Update PBXFileSystemSynchronizedGroupBuildPhaseMembershipExceptionSet to correctly support build_phase attribute pointing to PBXCopyFilesBuildPhase

[amorde]

Follow up to #985

If adding a target such as a static library with Objective-C language selected, the Copy Files build phase will have an exception set for the public headers.

Example:

/* Begin PBXFileSystemSynchronizedGroupBuildPhaseMembershipExceptionSet section */
	0782A2F32CCD962C007072F5 /* PBXFileSystemSynchronizedGroupBuildPhaseMembershipExceptionSet */ = {
		isa = PBXFileSystemSynchronizedGroupBuildPhaseMembershipExceptionSet;
		buildPhase = 0782A2EB2CCD962C007072F5 /* CopyFiles */;
		membershipExceptions = (
			Objc_iOS_StaticLibrary.h,
		);
	};
/* End PBXFileSystemSynchronizedGroupBuildPhaseMembershipExceptionSet section */

When attempting to read this project an error would occur:

[Xcodeproj] Type checking error: got `PBXCopyFilesBuildPhase` for attribute: Attribute `buildPhase` (type: `to_one`, classes: `["PBXSourcesBuildPhase"]`, owner class: `PBXFileSystemSynchronizedGroupBuildPhaseMembershipExceptionSet`)