[ObjC] Move `GPBEnumDescriptorFunc` to be private.

The type is internal to the generated code/runtime and isn't exposed
in any public api, so go ahead and move it into the Descriptor private
header.

PiperOrigin-RevId: 718383227

objectivec/GPBDescriptor_PackagePrivate.h

@@ -77,6 +77,9 @@ typedef struct GPBFileDescription {
   GPBFileSyntax syntax;
 } GPBFileDescription;
 
+// Fetches an EnumDescriptor.
+typedef GPBEnumDescriptor *(*GPBEnumDescriptorFunc)(void);
+
 // Describes a single field in a protobuf as it is represented as an ivar.
 typedef struct GPBMessageFieldDescription {
   // Name of ivar.

objectivec/GPBRuntimeTypes.h

@@ -17,11 +17,6 @@
  * */
 typedef BOOL (*GPBEnumValidationFunc)(int32_t);
 
-/**
- * Fetches an EnumDescriptor.
- * */
-typedef GPBEnumDescriptor *(*GPBEnumDescriptorFunc)(void);
-
 /**
  * Magic value used at runtime to indicate an enum value that wasn't know at
  * compile time.