SymbolComparator operator() is missing const

Clean up, no functionality change.

Test: build
Bug: 37752547
Change-Id: I7b6f368c0d0776f956a8b99353df7b23cbbc388d
diff --git a/tools/aapt2/ResourceParser.cpp b/tools/aapt2/ResourceParser.cpp
index a84c306..e2ee603 100644
--- a/tools/aapt2/ResourceParser.cpp
+++ b/tools/aapt2/ResourceParser.cpp
@@ -802,7 +802,7 @@
     }
 
     struct SymbolComparator {
-        bool operator()(const Attribute::Symbol& a, const Attribute::Symbol& b) {
+        bool operator()(const Attribute::Symbol& a, const Attribute::Symbol& b) const {
             return a.symbol.name.value() < b.symbol.name.value();
         }
     };