Make lib/strntol.c a stand-alone library

Make lib/strntol.c a stand-alone library

It needs to include strntol.h from local directory where strntol.c
is also located, to be able to use this as a stand-alone library,
which is useful for debugging purpose. In fact, most of the files
under lib/ directory do things this way.

--
 # cat ./test5.c
 #include <stdio.h>
 #include "lib/strntol.h"
 int main(void) {
         /* just to see if it compiles */
         strntol(NULL, 0, NULL, 0);
         return 0;
 }
 # gcc -Wall -g ./test5.c ./lib/strntol.c

Signed-off-by: Tomohiro Kusumi <tkusumi@tuxera.com>
Signed-off-by: Jens Axboe <axboe@fb.com>