Staging: wlags49_h2: add missing <linux/string.h> for strlen

On ia64, the build fails with incompatible implicit definition of strlen.
This patch adds the <linux/string.h> include to get the real prototype.

Signed-off-by: Jeff Mahoney <jeffm@suse.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>