Fix spurious printf format warnings with MinGW.

Correct MinGW format checking depends on the __MINGW_PRINTF_FORMAT
macro, which is defined in its stdio.h. cbfba7a0e9 moved the use of
that macro to before the inclusion of stdio.h, introducing spurious
"unknown conversion type character 'z'" and similar warnings.

defs.h

@@ -13,6 +13,7 @@
 
 #include <stddef.h>
 #include <stdint.h>
+#include <stdio.h>                     /* for __MINGW_PRINTF_FORMAT */
 #include <stdbool.h>
 
 #if defined _MSC_VER && _MSC_VER < 1800