readelf: Add stdint.h for C99 fixed size types

Upstream elftoolchain brings in stdint.h via an ELF header that we
do not use in FreeBSD.

Sponsored by:	The FreeBSD Foundation

contrib/elftoolchain/readelf/readelf.c

@@ -38,6 +38,7 @@
 #include <libelftc.h>
 #include <libgen.h>
 #include <stdarg.h>
+#include <stdint.h>
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>