Anim: fix printf format string

Use `PRId64` for `int64_t` values, to appearse both clang and gcc.

The warnings were introduced in efef8a201a

Pull Request: https://projects.blender.org/blender/blender/pulls/143979

source/blender/editors/animation/keyframing.cc

@@ -976,14 +976,14 @@ static wmOperatorStatus delete_key_vse_without_keying_set(bContext *C, wmOperato
     if (modified_strips.is_empty()) {
       BKE_reportf(op->reports,
                   RPT_WARNING,
-                  "No keyframes removed from %ld strip(s)",
+                  "No keyframes removed from %" PRId64 " strip(s)",
                   selected_strips_rna_paths.size());
       return OPERATOR_CANCELLED;
     }
 
     BKE_reportf(op->reports,
                 RPT_INFO,
-                "%ld strip(s) successfully had %ld keyframes removed",
+                "%" PRId64 " strip(s) successfully had %" PRId64 " keyframes removed",
                 modified_strips.size(),
                 modified_fcurves.size());
   }