Make oslib/linux-dev-lookup.c a stand-alone library

Make oslib/linux-dev-lookup.c a stand-alone library

It needs to include linux-dev-lookup.h from local directory where
linux-dev-lookup.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.

It also doesn't need to include os/os.h as it has no dependency.

--
 # cat ./test10.c
 #include <stdio.h>
 #include "oslib/linux-dev-lookup.h"
 int main(void) {
         /* just to see if it compiles */
         blktrace_lookup_device(NULL, NULL, 0, 0);
         return 0;
 }
 # gcc -Wall -g ./test10.c oslib/linux-dev-lookup.c

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