6lowpan: debugfs: add missing static

This patch solves the sparse warning:

net/6lowpan/debugfs.c:164:30: warning: symbol 'lowpan_ctx_pfx_fops' was
not declared. Should it be static?
net/6lowpan/debugfs.c:241:30: warning: symbol 'lowpan_context_fops' was
not declared. Should it be static?

Signed-off-by: Alexander Aring <aar@pengutronix.de>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>