Cleanup: Animation: mark function parameter as `const`

`fcurve_path_rename(..., rna_path, ...)` doesn't change `rna_path` at all,
so it can be marked as `const char *`.

No functional changes.

source/blender/editors/curve/editcurve.c

@@ -908,7 +908,7 @@ static bool curve_is_animated(Curve *cu)
 
 static void fcurve_path_rename(AnimData *adt,
                                const char *orig_rna_path,
-                               char *rna_path,
+                               const char *rna_path,
                                ListBase *orig_curves,
                                ListBase *curves)
 {