Update GPBProtocolBuffers.h

objectivec/GPBProtocolBuffers.h

diff --git a/objectivec/GPBProtocolBuffers.h b/objectivec/GPBProtocolBuffers.h
index 0c85259..677903e 100644
--- a/objectivec/GPBProtocolBuffers.h
+++ b/objectivec/GPBProtocolBuffers.h
@@ -41,6 +41,7 @@
 #import "GPBUnknownField.h"
 #import "GPBUnknownFieldSet.h"
 #import "GPBUtilities.h"
+#import "GPBWellKnownTypes.h"
 #import "GPBWireFormat.h"
 
 // Well-known proto types