Bluetooth: remove direct compilation of 6lowpan_iphc.c

It's now built as a separate utility module, and enabling BT selects
that module in Kconfig. This fixes:

net/ieee802154/built-in.o:(___ksymtab_gpl+lowpan_process_data+0x0): multiple definition of `__ksymtab_lowpan_process_data'
net/bluetooth/built-in.o:(___ksymtab_gpl+lowpan_process_data+0x0): first defined here
net/ieee802154/built-in.o:(___ksymtab_gpl+lowpan_header_compress+0x0): multiple definition of `__ksymtab_lowpan_header_compress'
net/bluetooth/built-in.o:(___ksymtab_gpl+lowpan_header_compress+0x0): first defined here
net/ieee802154/built-in.o: In function `lowpan_header_compress':
net/ieee802154/6lowpan_iphc.c:606: multiple definition of `lowpan_header_compress'
net/bluetooth/built-in.o:/home/swarren/shared/git_wa/kernel/kernel.git/net/bluetooth/../ieee802154/6lowpan_iphc.c:606: first defined here
net/ieee802154/built-in.o: In function `lowpan_process_data':
net/ieee802154/6lowpan_iphc.c:344: multiple definition of `lowpan_process_data'
net/bluetooth/built-in.o:/home/swarren/shared/git_wa/kernel/kernel.git/net/bluetooth/../ieee802154/6lowpan_iphc.c:344: first defined here
make[1]: *** [net/built-in.o] Error 1

(this change probably simply wasn't "git add"d to a53d34c3)

Fixes: a53d34c3 ("net: move 6lowpan compression code to separate module")
Fixes: 18722c24 ("Bluetooth: Enable 6LoWPAN support for BT LE devices")
Signed-off-by: Stephen Warren <swarren@nvidia.com>
Acked-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: David S. Miller <davem@davemloft.net>