lavd: export all symbols with av_ prefix

Signed-off-by: Lukasz Marek <lukasz.m.luki2@gmail.com>
(cherry picked from commit e493814d)
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavdevice/libavdevice.v

 LIBAVDEVICE_$MAJOR {
-        global: avdevice_*;
+        global: avdevice_*; av_*;
         local: *;
 };