When static linking programs using ffmpeg libraries, if linking against
libavutil, -ldrm is listed before -lavutil. This leads to linking failure
due to undefined reference of drmGetVersion() and drmFreeVersion().
This is why when pkg-config create libavutil.pc doesn't append -ldrm
after -lavutil.

Create LIBDRM=-ldrm in case libdrm is enabled and add $LIBDRM to
extralibs_avutil.
Signed-off-by: Giulio Benetti <giulio.benetti@micronovasrl.com>
Signed-off-by: James Almer <jamrial@gmail.com>