ci/freebsd: disable iconv

This actually started having a linking error a few days ago, but the ci
never actually actually exited 1 for some reason so it was never caught.
The actual commits where this started happening are unrelated (reverted
all of the them to be sure) and there's no other obvious things like a
meson update that would cause this. All I can assume is some other
package within BSD itself, but I can't be bothered to debug it. Anyways,
just disable iconv in the build for now so the CI stops failing.
Presumably there's something wrong in the environment with GNU's
libiconv and the built-in one clashing somehow. Also, sdl unbelievably
defines HAVE_ICONV in their public header so we have to disable that
too.

ci/build-freebsd.sh

@@ -9,11 +9,11 @@ meson setup build \
     --werror      \
     -Dlibplacebo:werror=false \
     -Dc_args="-Wno-error=deprecated -Wno-error=deprecated-declarations" \
+    -Diconv=disabled \
     -Dlibmpv=true \
     -Dlua=enabled \
     -Degl-drm=enabled \
     -Dopenal=enabled \
-    -Dsdl2=enabled \
     -Dsndio=enabled \
     -Dtests=true \
     -Dvdpau=enabled \