Fix clang compiler warning/error



git-svn-id: http://skia.googlecode.com/svn/trunk@8475 2bbb7eff-a529-9590-31e7-b0007b416f81
diff --git a/src/gpu/GrDrawState.cpp b/src/gpu/GrDrawState.cpp
index 6d7969f..aa9e692 100644
--- a/src/gpu/GrDrawState.cpp
+++ b/src/gpu/GrDrawState.cpp
@@ -146,7 +146,8 @@
                 int slVecCount = GrSLTypeVectorCount(effectSLType);
                 int attribVecCount = GrVertexAttribTypeVectorCount(attribType);
                 if (slVecCount != attribVecCount ||
-                    (-1 != slTypes[attribIndex] && slTypes[attribIndex] != effectSLType)) {
+                    (static_cast<GrSLType>(-1) != slTypes[attribIndex] && 
+                     slTypes[attribIndex] != effectSLType)) {
                     return false;
                 }
                 slTypes[attribIndex] = effectSLType;