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/src/gpu/gl/GrGLShaderBuilder.cpp b/src/gpu/gl/GrGLShaderBuilder.cpp
index c00cf91..0845fdd 100644
--- a/src/gpu/gl/GrGLShaderBuilder.cpp
+++ b/src/gpu/gl/GrGLShaderBuilder.cpp
@@ -172,7 +172,7 @@
                                                                      int count,
                                                                      const char** outName) {
     GrAssert(name && strlen(name));
-    static const uint32_t kVisibilityMask = kVertex_ShaderType | kFragment_ShaderType;
+    SkDEBUGCODE(static const uint32_t kVisibilityMask = kVertex_ShaderType | kFragment_ShaderType);
     GrAssert(0 == (~kVisibilityMask & visibility));
     GrAssert(0 != visibility);