Conditional compilation of PLATFORM_DESKTOP segments should also apply to PLATFORM_DESKTOP_LINUX segments

While checking the correctness of my last pull request, I had a version of libraysan.a compiled using `-DPLATFORM_DESKTOP`, which turned out to build correctly every example. However, I failed to notice that using `-DPLATFORM_DESKTOP_LINUX` excludes several raylib functions from the library. This pull request should fix that.

As a matter of style, I'm beginning to feel that preprocessing directives such as

    #if defined(PLATFORM_DESKTOP) || defined(PLATFORM_RPI) || defined(PLATFORM_DESKTOP_LINUX)

are starting to get a bit unwieldy, but I'm sticking to the overall coding style of libraysan. Maybe a shared `PLATFORM_DESKTOP` definition and separate `DESKTOP_WINDOWS` and `DESKTOP_LINUX` definitions would turn out to be more readable, but then again, maybe not.

Cheers!