x86/purgatory: Hard-code obj-y in Makefile

arch/x86/Kbuild guards the entire purgatory/ directory, and
CONFIG_KEXEC_FILE is bool type.

$(CONFIG_KEXEC_FILE) is always 'y' when this directory is being built.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Signed-off-by: Borislav Petkov <bp@suse.de>
Link: https://lore.kernel.org/r/20220725020812.622255-1-masahiroy@kernel.org