mapi: fix preprocessor check in u_current_destroy()

So it matches the preprocessor check around the u_current_init_tsd() code.

Reviewed-by: Matt Turner <mattst88@gmail.com>
Reviewed-by: José Fonseca <jfonseca@vmware.com>

src/mapi/u_current.c

@@ -125,7 +125,7 @@ static int ThreadSafe;
 void
 u_current_destroy(void)
 {
-#if defined(THREADS) && defined(_WIN32)
+#if defined(THREADS) && !defined(GLX_USE_TLS)
    u_tsd_destroy(&u_current_table_tsd);
    u_tsd_destroy(&u_current_context_tsd);
 #endif