net: Include appropriate header file in caif/caif_dev.c

Include appropriate header file net/caif/caif_dev.h in caif/caif_dev.c
because it has prototype declarations of function defined in
caif/caif_dev.c.

This eliminates the following file in caif/caif_dev.c:
net/caif/caif_dev.c:303:6: warning: no previous prototype for ‘caif_enroll_dev’ [-Wmissing-prototypes]

Signed-off-by: Rashika Kheria <rashika.kheria@gmail.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
Signed-off-by: David S. Miller <davem@davemloft.net>