use GetModuleHandleW() to retrieve kernel32.dll handle (bug #5390.)

SDL_systhread.c and SDL_syslocale.c used to call LoadLibrary() without
calling FreeLibrary() later.  GetModuleHandleW() should always succeed
because GetModuleHandleW() itself is imported from kernel32.dll and we
don't need to bother releasing it.

src/thread/windows/SDL_systhread.c

@@ -163,7 +163,7 @@ SDL_SYS_SetupThread(const char *name)
         static HMODULE kernel32 = 0;
 
         if (!kernel32) {
-            kernel32 = LoadLibraryW(L"kernel32.dll");
+            kernel32 = GetModuleHandleW(L"kernel32.dll");
             if (kernel32) {
                 pSetThreadDescription = (pfnSetThreadDescription) GetProcAddress(kernel32, "SetThreadDescription");
             }