x86/audit: Fix a -Wmissing-prototypes warning for ia32_classify_syscall()

Lift the prototype of ia32_classify_syscall() into its own header.

Signed-off-by: Benjamin Thiel <b.thiel@posteo.de>
Signed-off-by: Borislav Petkov <bp@suse.de>
Link: https://lkml.kernel.org/r/20200516123816.2680-1-b.thiel@posteo.de