um: Fix -Wmissing-prototypes warnings for text_poke*

The prototypes for text_poke* are declared in asm/text-patching.h
under arch/x86/include/. It's safe to include this header, as it's
UML-aware (by checking CONFIG_UML_X86).

This will address below -Wmissing-prototypes warnings:

arch/um/kernel/um_arch.c:461:7: warning: no previous prototype for ‘text_poke’ [-Wmissing-prototypes]
arch/um/kernel/um_arch.c:473:6: warning: no previous prototype for ‘text_poke_sync’ [-Wmissing-prototypes]

Signed-off-by: Tiwei Bie <tiwei.btw@antgroup.com>
Signed-off-by: Richard Weinberger <richard@nod.at>