Fixed MinGW build.

include/bx/sem.h

@@ -7,7 +7,6 @@
 #define BX_SEM_H_HEADER_GUARD
 
 #include "bx.h"
-#include "mutex.h"
 
 #if BX_CONFIG_SUPPORTS_THREADING
 
@@ -23,6 +22,8 @@
 #	include <limits.h>
 #endif // BX_PLATFORM_
 
+#include "mutex.h"
+
 namespace bx
 {
 	///