x86/xen: add prototypes for paravirt mmu functions

The paravirt MMU functions called via the PV_CALLEE_SAVE_REGS_THUNK()
macro can't be defined to be static, as the macro is generating a
function via asm() statement calling the paravirt MMU function.

In order to avoid warnings when specifying "-Wmissing-prototypes" for
the build, add local prototypes (there should never be any external
caller of those functions).

Reported-by: Arnd Bergmann <arnd@kernel.org>
Signed-off-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Link: https://lore.kernel.org/r/20230614073501.10101-2-jgross@suse.com


Signed-off-by: Juergen Gross <jgross@suse.com>