[ObjC] Output directive to disable clang-format on generated files.

Regenerate the WKTs accordingly.
diff --git a/objectivec/GPBDuration.pbobjc.m b/objectivec/GPBDuration.pbobjc.m
index 11ba5e5..bc61092 100644
--- a/objectivec/GPBDuration.pbobjc.m
+++ b/objectivec/GPBDuration.pbobjc.m
@@ -1,6 +1,8 @@
 // Generated by the protocol buffer compiler.  DO NOT EDIT!
 // source: google/protobuf/duration.proto
 
+// clang-format off
+
 #import "GPBProtocolBuffers_RuntimeSupport.h"
 #import "GPBDuration.pbobjc.h"
 
@@ -93,3 +95,5 @@
 #pragma clang diagnostic pop
 
 // @@protoc_insertion_point(global_scope)
+
+// clange-format on