Allow bootstrapping mkimg on macOS/Linux

On these systems the (u)int64_t typedefs will not be implicitly defined by the
previous includes, so include <stdint.h> in the header that uses uint64_t.

Reviewed By:	brooks
Differential Revision: https://reviews.freebsd.org/D23202

usr.bin/mkimg/image.h

@@ -29,6 +29,8 @@
 #ifndef _MKIMG_IMAGE_H_
 #define	_MKIMG_IMAGE_H_
 
+#include <stdint.h>
+
 typedef int64_t lba_t;
 
 int image_copyin(lba_t blk, int fd, uint64_t *sizep);