allow modprobe to load signed kernel modules

Modprobe requires this permission or the following denial will
prevent loading of signed kernel modules:

audit: type=1400 audit(27331649.656:4): avc:  denied  { search } for
pid=448 comm="modprobe" scontext=u:r:modprobe:s0 tcontext=u:r:kernel:s0
tclass=key permissive=0

Bug: 62256697
Test: Verified signed module loading on sailfish.
Change-Id: Idde41d1ab58e760398190d6686665a252f1823bb