deps.ffmpeg: Fix shared library CMake flag on macOS

The libdatachannel CMake does not have a ENABLE_SHARED flag. The correct
flag is BUILD_SHARED_LIBS.

Author: RytoEX

deps.ffmpeg/70-libdatachannel.zsh

@@ -34,7 +34,7 @@ config() {
 
   args=(
     ${cmake_flags}
-    -DENABLE_SHARED="${_onoff[(( shared_libs + 1 ))]}"
+    -DBUILD_SHARED_LIBS="${_onoff[(( shared_libs + 1 ))]}"
     -DUSE_MBEDTLS=1
     -DNO_WEBSOCKET=1
     -DNO_TESTS=1