6lowpan: Remove unused pointer in lowpan_header_create()

Commit 8df8c56a (6lowpan: Moving generic compression code into 6lowpan_iphc.c)
left pointer 'hdr' unused - remove it.

Detected by Coverity: CID 1164868.

Signed-off-by: Christian Engelmayer <cengelma@gmx.at>
Signed-off-by: David S. Miller <davem@davemloft.net>