Add header need for module maps.

If you make up a module map for Objective C protocol buffers, the compiler will complain about missing a declaration for GPBUnknownFieldSet which is used in this file.

objectivec/GPBUnknownField.m

@@ -32,6 +32,7 @@
 
 #import "GPBArray.h"
 #import "GPBCodedOutputStream_PackagePrivate.h"
+#import "GPBUnknownFieldSet.h"
 
 @implementation GPBUnknownField {
  @protected