Conditionally depend on stdio.h

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

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

ref:
https://github.com/rust-bitcoin/rust-secp256k1/blob/master/secp256k1-sys/depend/secp256k1.c.patch

This patch does the first part of #1095

src/util.h

@@ -13,9 +13,12 @@
 
 #include <stdlib.h>
 #include <stdint.h>
-#include <stdio.h>
 #include <limits.h>
 
+#ifndef USE_EXTERNAL_DEFAULT_CALLBACKS
+#include <stdio.h>
+#endif
+
 #define STR_(x) #x
 #define STR(x) STR_(x)
 #define DEBUG_CONFIG_MSG(x) "DEBUG_CONFIG: " x