Don't pass non-const refs.

Non-const refs are not allowed by style guide.
This was just an oversight.

Test: m bpf_existence_test
Change-Id: I8947118618dd5beef7f5643475d348f709799922

tests/mts/bpf_existence_test.cpp

@@ -92,7 +92,7 @@ void addAll(set<string>* a, const set<string>& b) {
     a->insert(b.begin(), b.end());
 }
 
-void removeAll(set<string>* a, const set<string> b) {
+void removeAll(set<string>* a, const set<string>& b) {
     for (const auto& toRemove : b) {
         a->erase(toRemove);
     }