package/udpcast: move comment around, to keep proper indentation

In Kconfig, when there is an item (like a comment) between a symbol and
its dependees, then the indentation is not in effect.

Move the symbol to the top of the file to guarantee the indentation in
menuconfig.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>