[media] staging/media: lirc_imon: add a __user annotation

This silences the following Sparse warnings:
lirc_imon.c:404:32: warning: incorrect type in argument 1 (different address spaces)
lirc_imon.c:404:32:    expected void const [noderef] <asn:1>*<noident>
lirc_imon.c:404:32:    got char const *buf
lirc_imon.c:117:28: warning: incorrect type in initializer (incompatible argument 2 (different address spaces))
lirc_imon.c:117:28:    expected long ( *write )( ... )
lirc_imon.c:117:28:    got long ( static [toplevel] *<noident> )( ... )

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>