MinGW: Avoid GCC -Wcast-function-type warnings on GetProcAddress

Using the same method as GNUlib and various other projects.
https://lists.gnu.org/archive/html/bug-gnulib/2018-08/msg00109.html

The warning is valid, but there's no way around it since the issue
comes from the Windows SDK. It's also harmless in this case.

Part of #29801.

platform/windows/os_windows.cpp

@@ -74,6 +74,11 @@ __declspec(dllexport) int AmdPowerXpressRequestHighPerformance = 1;
 #define WM_POINTERUPDATE 0x0245
 #endif
 
+#if defined(__GNUC__)
+// Workaround GCC warning from -Wcast-function-type.
+#define GetProcAddress (void *)GetProcAddress
+#endif
+
 typedef struct {
 	int count;
 	int screen;