audit: move the compat_xxx_class[] extern declarations to audit_arch.h

The comapt_xxx_class symbols aren't declared in anything that
lib/comapt_audit.c is including (arm64 build) which is causing
the following sparse warnings:

lib/compat_audit.c:7:10: warning: symbol 'compat_dir_class'
was not declared. Should it be static?
lib/compat_audit.c:12:10: warning: symbol 'compat_read_class'
was not declared. Should it be static?
lib/compat_audit.c:17:10: warning: symbol 'compat_write_class'
was not declared. Should it be static?
lib/compat_audit.c:22:10: warning: symbol 'compat_chattr_class'
was not declared. Should it be static?
lib/compat_audit.c:27:10: warning: symbol 'compat_signal_class'
was not declared. Should it be static?

Trying to fix this by chaning compat_audit.c to inclde <linux/audit.h>
does not work on arm64 due to compile errors with the extra includes
that changing this header makes. The simpler thing would be just to
move the definitons of these symbols out of <linux/audit.h> into
<linux/audit_arch.h> which is included.

Fixes: 4b58841149dca ("audit: Add generic compat syscall support")
Signed-off-by: Ben Dooks <ben.dooks@codethink.co.uk>
[PM: rewrite subject line, fixed line length in description]
Signed-off-by: Paul Moore <paul@paul-moore.com>