Conditionally include stdio.h

The header `stdio.h` is only needed in code ifdef guarded by
`USE_EXTERNAL_DEFAULT_CALLBACKS` and also (indirectly) by `VERIFY`, we
can therefore guard the include statement in the same manner.

The reason for doing this as that wasm builds downstream have to patch
in an empty `stdio.h` file in order to build because of the
unconditional include.

Author: tcharding

src/util.h

@@ -13,9 +13,16 @@
 
 #include <stdlib.h>
 #include <stdint.h>
-#include <stdio.h>
 #include <limits.h>
 
+#ifndef USE_EXTERNAL_DEFAULT_CALLBACKS
+#include <stdio.h>
+#endif
+
+#ifdef VERIFY
+#include <stdio.h>
+#endif
+
 #define STR_(x) #x
 #define STR(x) STR_(x)
 #define DEBUG_CONFIG_MSG(x) "DEBUG_CONFIG: " x