Title: Build failure on musl libc: BAUD_2000000 and related enum members missing #523

Describe the bug

Compiling LibSerial with a musl libc toolchain results in a compile error.

In SerialPort.h, high baudrate enum entries (BAUD_2000000, BAUD_2500000, BAUD_3000000, BAUD_3500000, BAUD_4000000) are conditionally defined only when __MAX_BAUD > B2000000.
Musl libc's termios only defines baud constants up to B1000000, so __MAX_BAUD is B1000000. Those enum members are not generated.

However, the GetBitRate() function inside SerialPort.cpp references those enum values via case statements without matching preprocessor guards. This causes GCC to throw an error because those enum identifiers do not exist in musl builds.

To Reproduce

1. Use an aarch64-linux-musl GCC toolchain
2. Build LibSerial from source
3. Compilation fails at SerialPort.cpp

Error message

/home/tiny/.mcpp/registry/data/xpkgs/compat-x-libserial/1.0.0/libserial-1.0.0/src/SerialPort.cpp:1226:24: error: 'BAUD_2000000' is not a member of 'LibSerial::BaudRate'; did you mean 'BAUD_1000000'?
1226 | case BaudRate::BAUD_2000000:
| ^\\\\\~
| BAUD_1000000
/home/tiny/.mcpp/registry/data/xpkgs/compat-x-libserial/1.0.0/libserial-1.0.0/src/SerialPort.cpp:1230:24: error: 'BAUD_2500000' is not a member of 'LibSerial::BaudRate'; did you mean 'BAUD_500000'?
1230 | case BaudRate::BAUD_2500000:
| ^\\\\\~
| BAUD_500000
/home/tiny/.mcpp/registry/data/xpkgs/compat-x-libserial/1.0.0/libserial-1.0.0/src/SerialPort.cpp:1234:24: error: 'BAUD_3000000' is not a member of 'LibSerial::BaudRate'; did you mean 'BAUD_1000000'?
1234 | case BaudRate::BAUD_3000000:
| ^\\\\\~
| BAUD_1000000
/home/tiny/.mcpp/registry/data/xpkgs/compat-x-libserial/1.0.0/libserial-1.0.0/src/SerialPort.cpp:1238:24: error: 'BAUD_3500000' is not a member of 'LibSerial::BaudRate'; did you mean 'BAUD_500000'?
1238 | case BaudRate::BAUD_3500000:
| ^\\\\\~
| BAUD_500000
/home/tiny/.mcpp/registry/data/xpkgs/compat-x-libserial/1.0.0/libserial-1.0.0/src/SerialPort.cpp:1242:24: error: 'BAUD_4000000' is not a member of 'LibSerial::BaudRate'; did you mean 'BAUD_1000000'?
1242 | case BaudRate::BAUD_4000000:
| ^\\\\\~
| BAUD_1000000

Expected behavior

The case branches for those high baudrates in GetBitRate() should be wrapped using the same preprocessor condition as the header file.

Proposed fix

Wrap the high baudrate case statements:

#ifdef __linux__
#if __MAX_BAUD > B2000000
        case BaudRate::BAUD_2000000:
            return 2000000;
        case BaudRate::BAUD_2500000:
            return 2500000;
        case BaudRate::BAUD_3000000:
            return 3000000;
        case BaudRate::BAUD_3500000:
            return 3500000;
        case BaudRate::BAUD_4000000:
            return 4000000;
#endif
#endif
Environment
- Target arch: aarch64
- Toolchain: GCC
- Libc: musl libc