libstdc++: Strip absolute paths from files shown in Doxygen docs

This avoids showing absolute paths from the expansion of
@srcdir@/libsupc++/ in the doxygen File List view.

libstdc++-v3/ChangeLog:

	* doc/doxygen/user.cfg.in (STRIP_FROM_PATH): Remove prefixes
	from header paths.

(cherry picked from commit 975e8e83)