ffmpeg: Add dl syslinks (#4255)

#4254

packages/f/ffmpeg/xmake.lua

@@ -57,9 +57,9 @@ package("ffmpeg")
             add_frameworks("AudioToolbox")
         end
     elseif is_plat("linux") then
-        add_syslinks("pthread")
+        add_syslinks("dl", "pthread")
     elseif is_plat("android") then
-        add_syslinks("android")
+        add_syslinks("dl", "android")
     end
 
     add_deps("nasm")