kbuild: fix O=.. build with arm

With a make O=... build kbuild would only create
the include2/asm symlink for archs that not yet
had moved headers to include/$ARCH/include

There is no longer any reason to avoid the symlink
for archs that has moved their headers so create it
unconditionally.

This fixes arm because kbuild checked for include/asm-$ARCH/errno.h
and that file was not present for arm but the platform files
are not yet moved.

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Cc: Russell King <rmk@arm.linux.org.uk>