Unbreak build.

Index: plugins/linux-v4l2/v4l2-helpers.h
--- plugins/linux-v4l2/v4l2-helpers.h.orig
+++ plugins/linux-v4l2/v4l2-helpers.h
@@ -17,7 +17,11 @@ along with this program.  If not, see <http://www.gnu.
 
 #pragma once
 
+#if defined(__OpenBSD__)
+#include <sys/videoio.h>
+#else
 #include <linux/videodev2.h>
+#endif
 #include <libv4l2.h>
 
 #include <obs-module.h>
