helpers: media_device: ignore lockf return value

[OldManYellsAtCloud]

When using gcc (with ~-Werror=unused-result), compilation fails, because the return value of lockf is ignored at one place.

To avoid this error, assign the return value to std::ignore.

The error message:

| ../sources/libpisp-1.2.1/src/helpers/media_device.cpp: In member function 'std::map<std::__cxx11::basic_string<char>, libpisp::helpers::DeviceFd>::iterator libpisp::helpers::MediaDevice::unlock(const std::string&)':
| ../sources/libpisp-1.2.1/src/helpers/media_device.cpp:251:14: error: ignoring return value of 'int lockf(int, int, __off64_t)' declared with attribute 'warn_unused_result' [-Werror=unused-result]
|   251 |         lockf(it->second.Get(), F_ULOCK, 0);
|       |         ~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
| cc1plus: all warnings being treated as errors