clang-tidy: run through performance checks

Found with performance*

Signed-off-by: Rosen Penev <rosenp@gmail.com>

src/xmpsidecar.cpp

@@ -119,7 +119,7 @@ namespace Exiv2 {
         return b;
     }
 
-    static bool matchi(const std::string key,const char* substr)
+    static bool matchi(const std::string& key, const char* substr)
     {
         return toLowerCase(key).find(substr) != std::string::npos;
     }