gst-ffmpeg: add --pkg-config to internal libav configure options

The internal copy of libav in gst-ffmpeg does not understand the
PKG_CONFIG env variable, so it throws a line like this at the end
of its configuration:

WARNING: .../output/host/usr/bin/i686-pc-linux-gnu-pkg-config not found, library detection may fail.

This patch adds the --pkg-config to let libav know where it is.

Signed-off-by: Danomi Manchego <danomimanchego123@gmail.com>
Tested-by: Luca Ceresoli <luca@lucaceresoli.net>
Reviewed-by: Luca Ceresoli <luca@lucaceresoli.net>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>