x86/syscalls: Add prototypes for C syscall callbacks

.. in order to fix a couple of -Wmissing-prototypes warnings.

No functional change.

 [ bp: Massage commit message and drop newlines. ]

Signed-off-by: Benjamin Thiel <b.thiel@posteo.de>
Signed-off-by: Borislav Petkov <bp@suse.de>
Link: https://lkml.kernel.org/r/20200123152754.20149-1-b.thiel@posteo.de