staging: easycap: use #ifndef __EASYCAP_H_ for header inclusion protection

use common #ifndef __EASYCAP_H_ instead of if (!defined(EASYCAP_H))
for protecting header from double inclusion

Cc: Mike Thomas <rmthomas@sciolus.org>
Signed-off-by: Tomas Winkler <tomas.winkler@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>