kdbus: Eliminate warning caused by lack of uapi/linux/kdbus.h inclusion

kdbus: Eliminate warning caused by lack of uapi/linux/kdbus.h inclusion

metadata.h references struct kdbus_pids which is defined in
uapi/linux/kdbus.h

Normally, kdbus/metadata.h is included as one of many other headers
that eventually include uapi/linux/kdbus.h at some point.

When included alone, it causes warning. Also when kdbus/connection.h
is included alone (e.g. in smack_lsm.c) , the same warning is shown
as it includes kdbus/metadata.h.

This patch adds missing inclusion.

Signed-off-by: Paul Osmialowski <p.osmialowsk@samsung.com>