Bluetooth: make sure 6LOWPAN_IPHC is built-in if needed

Bluetooth: make sure 6LOWPAN_IPHC is built-in if needed

Commit 975508879 "Bluetooth: make bluetooth 6lowpan as an option"
ensures that 6LOWPAN_IPHC is turned on when we have BT_6LOWPAN
enabled in Kconfig, but it allows building the IPHC code as
a loadable module even if the entire Bluetooth stack is built-in,
and that causes a link error.

We can solve that by moving the 'select' statement into CONFIG_BT,
which is a "tristate" option to enforce that 6LOWPAN_IPHC can
only be a module if BT also is a module.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>