Include -XX:DisableExplicitGC in the usage output.

Change-Id: I290d7b5762b616b02f59a653ac5a83388c299a93
diff --git a/vm/Init.c b/vm/Init.c
index ec0e136..d1ff157 100644
--- a/vm/Init.c
+++ b/vm/Init.c
@@ -119,6 +119,7 @@
     dvmFprintf(stderr, "  -Xgc:[no]postverify\n");
     dvmFprintf(stderr, "  -Xgc:[no]concurrent\n");
     dvmFprintf(stderr, "  -Xgc:[no]verifycardtable\n");
+    dvmFprintf(stderr, "  -XX:+DisableExplicitGC\n");
     dvmFprintf(stderr, "  -Xgenregmap\n");
     dvmFprintf(stderr, "  -Xverifyopt:[no]checkmon\n");
     dvmFprintf(stderr, "  -Xcheckdexsum\n");