SLURMSTEPD_MEMCHECK fix

This modifies code added in Slurm version 16.05 so that it works
properly with a change made in version 15.08 with respect to the
definition of SLURMSTEPD_MEMCHECK, see commit
c4d0d306

src/slurmd/slurmstepd/slurmstepd.c

@@ -379,7 +379,7 @@ _got_ack_from_slurmd(int sock)
 {
 	/* If running under valgrind/memcheck, this pipe doesn't work correctly
 	 * so just skip it. */
-#ifndef SLURMSTEPD_MEMCHECK
+#if (SLURMSTEPD_MEMCHECK == 0)
 	int ok;
 	safe_read(sock, &ok, sizeof(int));
 	return;