Make lib/zipf.c a stand-alone library

lib/zipf.c not having dependency on fio header (log.h) enables it
to be a stand-alone library, which is useful for debugging purpose.
In fact, most of the files under lib/ directory do things this way.

lib/zipf.c doesn't use log.h (log(3) is apparently from -lm).

This requires the previous commit.

--
 # cat ./test8.c
 #include <stdio.h>
 #include "lib/zipf.h"
 int main(void) {
         /* just to see if it compiles */
         zipf_init(NULL, 0, 0, 0);
         return 0;
 }
 # gcc -Wall -g -lm -DBITS_PER_LONG=64 ./test8.c ./lib/zipf.c ./lib/rand.c ./lib/pattern.c ./lib/strntol.c

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