make: Provide mandatory options in cflags-y variable

In case if CFLAGS is overriden from command line we don't
see our headers anymore. So provide mandatory options in
ccflags-y variable to fix that.

https://bugzilla.openvz.org/show_bug.cgi?id=2521



Reported-by: Jan Engelhardt <jengelh@inai.de>
Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
Tested-by: Alexander Kartashov <alekskartashov@parallels.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>