cmake-qemu.bbclass: fix if criterion

It always executes the scripts whether 'qemu-usermode' in
'MACHINE_FEATURES' or not. Fix the criterion to make it work.

Signed-off-by: Kai Kang <kai.kang@windriver.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>