table/table_test.cc: pass func parameter by reference

Fix for:

[table/table_test.cc:1218]: (performance) Function parameter
 'prefix' should be passed by reference.

Signed-off-by: Danny Al-Gaaf <danny.al-gaaf@bisect.de>

table/table_test.cc

@@ -1216,7 +1216,7 @@ static std::string RandomString(Random* rnd, int len) {
   return r;
 }
 
-void AddInternalKey(TableConstructor* c, const std::string prefix,
+void AddInternalKey(TableConstructor* c, const std::string& prefix,
                     int suffix_len = 800) {
   static Random rnd(1023);
   InternalKey k(prefix + RandomString(&rnd, 800), 0, kTypeValue);