package/gstreamer: 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.

In this case, it means the gstremaer plugins are not indented below
the main gstreamer prompt.

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