rt-tests: cyclictest: Fix warning: ‘cpu’ may be used uninitialized

Fix warning 'cpu' maybe uninitialized by giving it a default value in
the switch statement. Note, this is a false positive, but documenting
the default value in the switch statement isn't a bad thing.

Signed-off-by: John Kacur <jkacur@redhat.com>

src/cyclictest/cyclictest.c

@@ -1897,6 +1897,7 @@ int main(int argc, char **argv)
 				printf("Thread %d using cpu %d.\n", i, cpu);
 			break;
 		case AFFINITY_USEALL: cpu = i % max_cpus; break;
+		default: cpu = -1;
 		}
 
 		node = -1;