kexec: build multiboot2 for i386

This addresses the following compilation issues when building for i386.

 kexec/arch/i386/kexec-x86.c:39:22: error: 'multiboot2_x86_probe' undeclared here (not in a function); did you mean 'multiboot_x86_probe'?
   { "multiboot2-x86", multiboot2_x86_probe, multiboot2_x86_load,
                       ^~~~~~~~~~~~~~~~~~~~
                       multiboot_x86_probe
 kexec/arch/i386/kexec-x86.c:39:44: error: 'multiboot2_x86_load' undeclared here (not in a function); did you mean 'multiboot_x86_load'?
   { "multiboot2-x86", multiboot2_x86_probe, multiboot2_x86_load,
                                             ^~~~~~~~~~~~~~~~~~~
                                             multiboot_x86_load
 kexec/arch/i386/kexec-x86.c:40:4: error: 'multiboot2_x86_usage' undeclared here (not in a function); did you mean 'multiboot_x86_usage'?
     multiboot2_x86_usage },
     ^~~~~~~~~~~~~~~~~~~~
     multiboot_x86_usage
 make: *** [Makefile:114: kexec/arch/i386/kexec-x86.o] Error 1
 make: *** Waiting for unfinished jobs....

Signed-off-by: Chris Packham <chris.packham@alliedtelesis.co.nz>
Signed-off-by: Simon Horman <horms@verge.net.au>