Fix libstdc++ lstat missing return type for Windows target.

Fix libstdc++ lstat missing return type for Windows target.

Building for i686-mingw32 target (with some local changes) produced an
error

error: ISO C++ forbids declaration of 'lstat' with no type [-fpermissive]

in libstdc++-v3/src/filesystem/ops-common.h.  This patch adds the
missing return type.  OK to commit (trunk and GCC 9 branch)?

Note 1: I haven't run the testsuite with this patch, but it fixes the
build failure I see (in sources with other local changes).

Note 2: I don't know why this hasn't produced build failures for other
people, but the missing return type looks wrong in any case even if
other local changes are for some reason needed for it to produce an
error.

	* src/filesystem/ops-common.h [_GLIBCXX_FILESYSTEM_IS_WINDOWS]
	(std::filesystem::__gnu_posix::lstat): Add return type.

From-SVN: r274885