audit: uninitialize variable audit_sig_sid

Checkpatch tool reports

"ERROR: do not initialise globals/statics to 0"

To fix this, audit_sig_sid is uninitialized
As this is stored in the .bss section,
the compiler can initialize the variable automatically.

Signed-off-by: Jules Irenge <jbi.octave@gmail.com>
Signed-off-by: Paul Moore <paul@paul-moore.com>