v4l2loopback: fix build in kernel 3.18

pkgs/os-specific/linux/v4l2loopback/default.nix

···
     sed -i '/depmod/d' Makefile
     export PATH=${kmod}/sbin:$PATH
   '';
+
+  patches = [ ./kernel-3.18-fix.patch ];
   
   buildInputs = [ kmod ];
   

pkgs/os-specific/linux/v4l2loopback/kernel-3.18-fix.patch

···
+From 21195cd6d1ff767a271359dfa7d201078f766611 Mon Sep 17 00:00:00 2001
+From: tatokis <[e-mail]>
+Date: Mon, 24 Nov 2014 16:28:33 +0200
+Subject: [PATCH] Updated v4l2loopback.c to compile on >= 3.18 kernel
+
+---
+ v4l2loopback.c | 9 +++++++--
+ 1 file changed, 7 insertions(+), 2 deletions(-)
+
+diff --git a/v4l2loopback.c b/v4l2loopback.c
+index bb228bb..67f6ed4 100644
+--- a/v4l2loopback.c
++++ b/v4l2loopback.c
+@@ -498,10 +498,15 @@ static ssize_t attr_store_maxopeners(struct device *cd,
+ {
+ 	struct v4l2_loopback_device *dev = NULL;
+ 	unsigned long curr = 0;
+-
++	
++	#if LINUX_VERSION_CODE >= KERNEL_VERSION(3,18,0)
++	if (kstrtoul(buf, 0, &curr))
++		return -EINVAL;
++	#else
+ 	if (strict_strtoul(buf, 0, &curr))
+ 		return -EINVAL;
+-
++	#endif
++	
+ 	dev = v4l2loopback_cd2dev(cd);
+ 
+ 	if (dev->max_openers == curr)