Remove mutable declaration of reference member

Mutable on reference members is meaningless.  gcc>=4.6 does not allow
it.

Change-Id: I87c10cd7b5d6a15f46ca20465882e7a940e850bf

libpp/format_output.h

@@ -91,7 +91,7 @@ class formatter {
 		symbol_entry const & symbol;
 		sample_entry const & sample;
 		size_t pclass;
-		mutable counts_t & counts;
+		counts_t & counts;
 		extra_images const & extra;
 		double diff;
 	};