kmod: fix build failure on gcc < 4.6 caused by _Static_assert()

This commit introduces a patch to kmod that ensures _Static_assert()
is only used if available. The patch has been submitted upstream.

Fixes:

  http://autobuild.buildroot.net/results/9daf0f46642020591731e20d3bf9041ff6259846/



Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>