gold: Move sym declaration just before use

Move sym declaration just before use to avoid -Wmaybe-uninitialized
warning from GCC 11.

	PR gold/27097
	* incremental.cc (Sized_relobj_incr::do_add_symbols): Move sym
	declaration just before use.
	(Sized_incr_dynobj::do_add_symbols): Likewise.
	* plugin.cc (Sized_pluginobj::do_add_symbols): Likewise.