libstdc++: Fix sizeof(wide-string)-1 bug in std::regex test

libstdc++: Fix sizeof(wide-string)-1 bug in std::regex test

This uses sizeof on a wide string to get the length, which is wrong
because each wchar_t is more than one byte. This was presumably copied
from a narrow char test.

libstdc++-v3/ChangeLog:

	* testsuite/28_regex/basic_regex/assign/wchar_t/pstring.cc: Use
	wcslen(cs) instead of sizeof(cs)-1.

Reviewed-by: Tomasz Kamiński <tkaminsk@redhat.com>