audit: Fix compilation issue

Fixes following compilation error:
kernel/auditfilter.c:426:3: warning: this decimal constant
is unsigned only in ISO C90 [enabled by default]
error, forbidden warning: auditfilter.c:426

Change-Id: Ica9d5ba72b699789b60f7226f9518c535d7b437e
Signed-off-by: Satya Durga Srinivasu Prabhala <satyap@codeaurora.org>