mkswap: fix compiler warning [-Wunused-variable]

Signed-off-by: Karel Zak <kzak@redhat.com>
build-sys: fstrim depends on libmount

Signed-off-by: Karel Zak <kzak@redhat.com>
tests: update build-sys tests

Signed-off-by: Karel Zak <kzak@redhat.com>