libstdc++: Suppress -Waddress warning in tzdb.cc [PR108228]

For some tarets the weak symbol is always defined, so we get a warning
that its address is never null. The warning isn't useful in this case,
so suppress it.

libstdc++-v3/ChangeLog:

	PR libstdc++/108228
	* src/c++20/tzdb.cc (zoneinfo_dir): Add diagnostic pragma.