add missing type shmatt_t in sys/shm.h

this type is not really intended to be used; it's just there to allow
implementations to choose the type for the shm_nattch member of
struct shmid_sh, presumably since historical implementations disagreed
on the type. in any case, it needs to be there, so now it is.

include/sys/shm.h

@@ -47,6 +47,8 @@ struct shm_info {
 #endif
 };
 
+typedef unsigned long shmatt_t;
+
 void *shmat(int, const void *, int);
 int shmctl(int, int, struct shmid_ds *);
 int shmdt(const void *);