Fix: Compiler warning after recent refactor

`rna_path` was passed to `BKE_reportf`, while it should be `rna_path.path.c_str()`

source/blender/animrig/intern/keyframing.cc

@@ -741,7 +741,7 @@ int clear_keyframe(Main *bmain, ReportList *reports, ID *id, const RNAPath &rna_
         RPT_ERROR,
         "Could not clear keyframe, as RNA path is invalid for the given ID (ID = %s, path = %s)",
         id->name,
-        rna_path);
+        rna_path.path.c_str());
     return 0;
   }