Include stdio.h and stdlib.h explicitly in secp256k1.c

src/secp256k1.c

@@ -44,6 +44,8 @@
 } while(0)
 
 #ifndef USE_EXTERNAL_DEFAULT_CALLBACKS
+#include <stdlib.h>
+#include <stdio.h>
 static void default_illegal_callback_fn(const char* str, void* data) {
     (void)data;
     fprintf(stderr, "[libsecp256k1] illegal argument: %s\n", str);