libbacktrace: Fix clang-tidy warning

* warning: overloaded 'operator{++,--}' returns a reference instead of a constant object type [cert-dcl21-cpp]

Change-Id: I8a904622b30dc70d17483b87748c2ea541e50c46

libbacktrace/include/backtrace/BacktraceMap.h

@@ -79,7 +79,7 @@ public:
       index_++;
       return *this;
     }
-    iterator& operator++(int increment) {
+    const iterator operator++(int increment) {
       index_ += increment;
       return *this;
     }
@@ -87,7 +87,7 @@ public:
       index_--;
       return *this;
     }
-    iterator& operator--(int decrement) {
+    const iterator operator--(int decrement) {
       index_ -= decrement;
       return *this;
     }