audit: Remove "weak" from audit_classify_compat_syscall() declaration

There's only one audit_classify_compat_syscall() definition, so it doesn't
need to be weak.

Remove the "weak" attribute from the audit_classify_compat_syscall()
declaration.

Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Richard Guy Briggs <rgb@redhat.com>
CC: AKASHI Takahiro <takahiro.akashi@linaro.org>