xbmc: fix missing libraries from rpi-userland

When rpi-userland libraries are built as shared-objects, not all needed
libraries are passed in the LD_FLAGS, leading to failure at linkage.

To avoid this issue, set the LIBS variable content in accordance with the
INCLUDES variable value.

The commit 5e3b1f31 currently works
around this issue by forcing rpi-userland to be statically linked,
whereas it is actually a LIBS flags problem in xbmc package.

A follow-up patch fixes rpi-userland package.

Reported-by: Maxime Hadjinlian <maxime.hadjinlian@gmail.com>
Signed-off-by: Samuel Martin <s.martin49@gmail.com>
Cc: Maxime Hadjinlian <maxime.hadjinlian@gmail.com>
Cc: "Yann E. MORIN" <yann.morin.1998@free.fr>
Tested-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Acked-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Tested-by: "Maxime Hadjinlian" <maxime.hadjinlian@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>