Silence -Wformat-nonliteral warning with clang.

lib/util.c

@@ -41,6 +41,10 @@
 #endif
 #include "xbps_api_impl.h"
 
+#ifdef __clang__
+#pragma clang diagnostic ignored "-Wformat-nonliteral"
+#endif
+
 /**
  * @file lib/util.c
  * @brief Utility routines