memory/patcher: fix compiler warning

syscall() returns a long, but we are invoking shmat(), which returns
a void*.

Signed-off-by: Maxwell Coil <mcoil@nd.edu>
(cherry picked from commit 52a9cce6f3dcd87e9ae66177398b60b9317e9339)

opal/mca/memory/patcher/memory_patcher_component.c

@@ -440,7 +440,7 @@ static void *_intercept_shmat(int shmid, const void *shmaddr, int shmflg)
 
     if (!original_shmat) {
 #if defined(SYS_shmat)
-        result = memory_patcher_syscall(SYS_shmat, shmid, shmaddr, shmflg);
+        result = (void*) memory_patcher_syscall(SYS_shmat, shmid, shmaddr, shmflg);
 #else // IPCOP_shmat
         unsigned long ret;
         ret = memory_patcher_syscall(SYS_ipc, IPCOP_shmat,