Fix build of ss-resourcemanager

The ss-resourcemanager (and agl-service-ss-resourcemanager)
requires to include the kernel header and it was planned
that STAGING_KERNEL_BUILDDIR to be passed through
KERNEL_HEADER_DIR vairable.

However, from the latest recipe, it seems that another
solution has been adopted that does not use KERNEL_HEADER_DIR
and it is not defined.

As a result, this undefined variable destroys the include paths,
which causes the compilation error.

Signed-off-by: Tadao Tanikawa <tanikawa.tadao@jp.panasonic.com>
Change-Id: Ifecc12bf5aa5ca5eaf9b51f99e1583d76ddb1908