Add conditional compilation for platform units

This is a step towards being able to compile GLFW manually without
needing to duplicate a lot of platform- or OS-specific logic.

src/win32_time.c

@@ -29,6 +29,7 @@
 
 #include "internal.h"
 
+#if defined(GLFW_BUILD_WIN32_TIMER)
 
 //////////////////////////////////////////////////////////////////////////
 //////                       GLFW platform API                      //////
@@ -51,3 +52,5 @@ uint64_t _glfwPlatformGetTimerFrequency(void)
     return _glfw.timer.win32.frequency;
 }
 
+#endif // GLFW_BUILD_WIN32_TIMER
+