audit: change unnecessary globals into statics

Variables sig_pid, audit_sig_uid and audit_sig_sid
are only used in the audit.c file across the kernel
Hence it appears no reason for declaring them as globals
This patch removes their global declarations from the .h file
and change them into static in the .c file.

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