buildroot

package/rpi-firmware: install missing library

The vcdbg utility is linked to a few libraries, which so far were all
provided by the rpi-userland package.

But a not-so-recent bump of rpi-firmware pulled in a vcdbg that is
linked to an additional library, which is not privided by rpi-userland,
so we must install it.

Reported-by: cluelessperson on #buildroot
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Maxime Hadjinlian <maxime.hadjinlian@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>