linuxkpi: Allow BUILD_BUG_ON in if statements without braces

I got a compilation failure in virtio-gpu without this change.

Reviewed By: linuxkpi, manu, bz, hselasky
Differential Revision: https://reviews.freebsd.org/D32366

(cherry picked from commit 6d15ccde4d9579ad2a77f93630695804a7efcbc4)