Add symbol versioning for shared libraries

Based on patch by Reinhard Tartler <siretart tauware de>

Originally committed as revision 30315 to svn://svn.mplayerhq.hu/mplayer/trunk/libswscale

libswscale/libswscale.v

+LIBSWSCALE_$MAJOR {
+        global: swscale_*; sws_*; ff_*;
+        local: *;
+};