libstdc++: Fix preprocessor check for bfloat16_t in <numbers> [PR126849]

libstdc++-v3/ChangeLog:

	PR libstdc++/126849
	* include/std/numbers: Fix preprocessor check for
	std::bfloat16_t support.

(cherry picked from commit 92bcc711fafc026e0cc2f48b7f3ab23657787a8c)
1 file changed