Fixed ISO C99 compatibility

SDL now builds with gcc 7.2 with the following command line options:
-Wall -pedantic-errors -Wno-deprecated-declarations -Wno-overlength-strings --std=c99

src/filesystem/windows/SDL_sysfilesystem.c

@@ -50,7 +50,7 @@ SDL_GetBasePath(void)
         return NULL;
     }
 
-    pGetModuleFileNameExW = (GetModuleFileNameExW_t)GetProcAddress(psapi, "GetModuleFileNameExW");
+    *(void**)&pGetModuleFileNameExW = GetProcAddress(psapi, "GetModuleFileNameExW");
     if (!pGetModuleFileNameExW) {
         WIN_SetError("Couldn't find GetModuleFileNameExW");
         FreeLibrary(psapi);