audit: fix build error when asm/syscall.h does not exist

avr32 does not have an asm/syscall.h file.  We need the
syscall_get_arch() definition from that file for all arch's which
support CONFIG_AUDITSYSCALL.  Obviously avr32 is not one of those
arch's.  Move the include inside the CONFIG_AUDITSYSCALL such that we
only do the include if we need the results.

When the syscall_get_arch() call is moved inside __audit_syscall_entry()
this include can be dropped entirely.  But that is going to require some
assembly changes on x86* in a patch that is not ready for the tree...

Reported-by: Paul Gortmaker <paul.gortmaker@windriver.com>
Signed-off-by: Eric Paris <eparis@redhat.com>