PR31941 objcopy --globalize-symbol

I think FILE symbols are special, and I can't see why anyone would
want them to be made global.  The fact that no one has reported this
bug since commit 7b4a0685e80a in 2005 supports that claim.

	PR 31941
	* objcopy.c (filter_symbols): Don't allow BSF_FILE symbols to
	be made global.