Updated v4l2loopback.c to compile on >= 3.18 kernel

Attempting to compile the module under 3.18 results in
```
/v4l2loopback/v4l2loopback.c:502:2: error: implicit declaration of function ‘strict_strtoul’ [-Werror=implicit-function-declaration]
  if (strict_strtoul(buf, 0, &curr))
  ^
cc1: some warnings being treated as errors
```
It seems that strict_strtoul has been removed from kernel.h on 3.18 and is possible to replace it with kstrtoul.