logging/backend: Change type of filename from string to const char*

This was suggested by degasus in https://github.com/yuzu-emu/yuzu/pull/3326 in order to avoid an unnecessary copy.

src/common/logging/backend.h

@@ -24,7 +24,7 @@ struct Entry {
     std::chrono::microseconds timestamp;
     Class log_class;
     Level log_level;
-    std::string filename;
+    const char* filename;
     unsigned int line_num;
     std::string function;
     std::string message;