md5: Fix cross-build after c2870e576b

On macOS and Linux the current set of headers do not end up pulling in
sys/param.h, causing MAXPATHLEN to not be defined and the build to fail.

(cherry picked from commit cab31e0e21)

sbin/md5/md5.c

@@ -21,6 +21,7 @@
 __FBSDID("$FreeBSD$");
 
 #include <sys/types.h>
+#include <sys/param.h>
 #include <sys/time.h>
 #include <sys/resource.h>
 #include <err.h>