util: Prevent implicit declaration of function getenv.

With MinGW cross compilation.

Reviewed-by: Brian Paul <brianp@vmware.com>
Reviewed-by: Roland Scheidegger <sroland@vmware.com>
Acked-by: Eric Engestrom <eric@engestrom.ch>

src/util/os_misc.c

@@ -38,6 +38,7 @@
 #endif
 #include <windows.h>
 #include <stdio.h>
+#include <stdlib.h>
 
 #else