uhttpd: fix time_t type mismatch on 32bit systems

The previous update introducing LFS support unconditionally changed the
sprintf() pattern used to print the file modification time to use PRIx64.

Explicitely convert the st_mtime member of the stat struct to uint64_t in
order to avoid type mismatch errors when building for non-64bit targets.

Signed-off-by: Jo-Philipp Wich <jow@openwrt.org>

SVN-Revision: 44138