swresample/libswresample.v: hide ff_*

Found-by: Hendrik Leppkes <h.leppkes@gmail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libswresample/libswresample.v

 LIBSWRESAMPLE_$MAJOR {
-        global: swr_*; ff_*; swresample_*;
+        global: swr_*; swresample_*;
         local: *;
 };