libstdc++: more #pragma diagnostic

The CI saw failures on 17_intro/headers/c++2011/parallel_mode.cc due to
-Wdeprecated-declarations warnings in some parallel/ headers.

libstdc++-v3/ChangeLog:

	* include/parallel/base.h: Suppress -Wdeprecated-declarations.
	* include/parallel/multiseq_selection.h: Likewise.