lib/string/: strerrno(): Use statement expression to perform lvalue conversion

lib/string/: strerrno(): Use statement expression to perform lvalue conversion

Compound literals are lvalues.  This means it's possible to take their
address.  That is, it would be possible (albeit nonsensical) to do

	&strerrno();

It is also possible to assign to them (albeit also nonsensical):

	strerrno() = NULL;

The statement expression performs lvalue conversion, which turns the
lvalue into an "rvalue", as expected, and disallows all those issues.

Signed-off-by: Alejandro Colomar <alx@kernel.org>