The gio library detection code no longer works, due to a missing $ in
front of the gio_libs reference. Make the string be $gio_libs.

Fixes: 76346b62 ("configure: Test that gio libs from pkg-config
work")

Signed-off-by: Bruce Rogers <brogers@suse.com>
Message-Id: <20201103145121.668865-1-brogers@suse.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>