Add symbol versioning for shared libraries

Based on patch by Reinhard Tartler <siretart tauware de>

git-svn-id: file:///var/local/repositories/mplayer/trunk/libswscale@30315 b3059339-0415-0410-9bf9-f77b7e298cf2

libswscale.v

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