libstdc++: Fix copyright notice to use usual form [PR108413]

libstdc++-v3/ChangeLog:

	PR libstdc++/108413
	* include/c_compatibility/stdatomic.h: Change copyright line to
	be consistent with other headers contributed under DCO terms.
	* include/std/expected: Add full stop to copyright line.
	* src/c++20/tzdb.cc: Likewise.