staging:dgap: Fix externs should be avoided in .c files as reported by checkpatch

This patch fixes "externs should be avoided in .c files"
in dgap.c as reported by checkpatch

Signed-off-by: Mark Hounschell <markh@compro.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>