fix warning from scalar --> int32 conversion

BUG=

Review URL: https://codereview.appspot.com/7065050

git-svn-id: http://skia.googlecode.com/svn/trunk@7061 2bbb7eff-a529-9590-31e7-b0007b416f81
diff --git a/gm/bigmatrix.cpp b/gm/bigmatrix.cpp
index 31c0a38..deaf448 100644
--- a/gm/bigmatrix.cpp
+++ b/gm/bigmatrix.cpp
@@ -41,6 +41,7 @@
 
         bool success = m.invert(&m);
         SkASSERT(success);
+        (void) success; // silence compiler :(
 
         SkPath path;