Make oslib/strlcat.c a stand-alone library

Make oslib/strlcat.c a stand-alone library

It needs to include strlcat.h from local directory where strlcat.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 oslib/ directory do things this way.

--
 # cat ./test9.c
 #include <stdio.h>
 #include "oslib/strlcat.h"
 int main(void) {
         /* just to see if it compiles */
         strlcat(NULL, NULL, 0);
         return 0;
 }
 # gcc -Wall -g ./test9.c ./oslib/strlcat.c

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