util: include string.h in u_string.h

To fix MinGW compiler warning about missing strlen() prototype.
Not sure how I missed this when fixing the malloc() / stdlib.h issue.

Reviewed-by: Charmaine Lee <charmainel@vmware.com>

src/util/u_string.h

@@ -41,6 +41,7 @@
 #include <stdlib.h>
 #include <stddef.h>
 #include <stdarg.h>
+#include <string.h>
 
 #include "util/macros.h" // PRINTFLIKE