vbox: fix link error with 'gcc -Og'

vbox: fix link error with 'gcc -Og'

With the new CONFIG_CC_OPTIMIZE_FOR_DEBUGGING option, we get a link
error in the vboxguest driver, when that fails to optimize out the
call to the compat handler:

drivers/virt/vboxguest/vboxguest_core.o: In function `vbg_ioctl_hgcm_call':
vboxguest_core.c:(.text+0x1f6e): undefined reference to `vbg_hgcm_call32'

Another compile-time check documents better what we want and avoids
the error.

Acked-by: Randy Dunlap <rdunlap@infradead.org>
Tested-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>