testsuite: shmget returns an int

testsuite: shmget returns an int

Stops the compilier (correctly) complaining:

lib/test_shm.c: In function ‘main’:
lib/test_shm.c:65:23: warning: format ‘%llx’ expects argument of type ‘long long unsigned int’, but argument 2 has type ‘int’ [-Wformat=]
   65 |     printf("SHMID: %llx\n", shm_id);
      |                    ~~~^     ~~~~~~
      |                       |     |
      |                       |     int
      |                       long long unsigned int

shm_id is an int which is what shmget() returns.

Strangely pmap has always scanned this in as a llx even though the maps
"inode" column is the same number that shmget() returns.

Signed-off-by: Craig Small <csmall@dropbear.xyz>