[rcore][GLFW] Fix warning on windows in MSVC

[JeffM2501]

There is an error with the forward declaration of the winMM functions when building on MSVC. Inconsistent DLL linkage.
This is not a critical warning, so this PR disables it.
This PR also removes the timeBeginPeriod declaration as it is no longer needed.

[JeffM2501]

AFAIK timeBeginPeriod may still be necessary, as its operation changed on Windows 10, version 2004 (see Remarks section, third paragraph, of the documentation), which was on May 2020, not that long ago. There's a lot of hardware still running older versions of Windows (e.g.: automation, industry, etc).

It's not called in that file, thus it does not need to be forward declared, it'll still be linked by the lib if some other internal file uses it, like GLFW itself.

[raysan5]

@JeffM2501 thanks for the review!