kexec: Include compat/kexec.h early

kexec: Include compat/kexec.h early

Include compat/kexec.h at the top of xen/common/kexec.c to allow
kexec_get_range_compat() to be compiled outside of #ifndef COMPAT.

This will slightly simplify the explicit creation of other _compat()
functions and is needed for the eventual removal of
xen/common/compat/kexec.c

Note that the inclusion of compat/kexec.h needs to be protected
by #ifdef CONFIG_COMPAT, as the file doesn't exist otherwise.

Signed-off-by: Simon Horman <horms@verge.net.au>