Fix "cast from pointer to integer of different size" warning on OpenBSD

Fix "cast from pointer to integer of different size" warning on OpenBSD

OpenBSD has below compiler warning due to pthread_t being typedef'd
to struct pthread*.

 os/os-openbsd.h: In function 'gettid':
 os/os-openbsd.h:71: warning: cast from pointer to integer of different size

--
 # uname
 OpenBSD
 # grep pthread_t /usr/include -rI | grep typedef | head -1
 /usr/include/pthread.h:typedef struct   pthread                 *pthread_t;
 # cat ./p1.c
 #include <stdio.h>
 #include <pthread.h>
 int main(void) {
         pthread_t id = 1234;
         printf("%d\n", (int)id);
         return 0;
 }
 # gcc -Wall -g ./p1.c
 ./p1.c: In function 'main':
 ./p1.c:4: warning: initialization makes pointer from integer without a cast
 ./p1.c:5: warning: cast from pointer to integer of different size
 # ./a.out
 1234

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