DRTVWR-418: Fix a (correct) clang correctness complaint.

LLStatGraph::Threshold has an operator<(const Threshold& other) -- but because
the method itself wasn't marked const, it could only be used on a non-const
instance. This change fixes a case when it was applied to const instances.

indra/llui/llstatgraph.h

@@ -126,7 +126,7 @@ class LLStatGraph : public LLView
 
 		F32 mValue;
 		LLUIColor mColor;
-		bool operator <(const Threshold& other)
+		bool operator <(const Threshold& other) const
 		{
 			return mValue < other.mValue;
 		}