Fixes for 4.20 suggested by @ddscentral

[tommyjcarpenter]

thanks to @ddscentral in this thread: dorimanx#136

[tommyjcarpenter]

This was tested on Arch's latest kernel 4.20 this morning (dec 31 2018)

[barrybingo]

Thanks @tommyjcarpenter, I just updated my arch kernel and ran into the same compile error.
I've put some precompiler conditions in and commited so will close this pull request.
Cheers.

[tommyjcarpenter]

@barrybingo awesome thanks. Question, is your branch ever going to gert merged into the source? Otherwise, maybe your branch should be the source of the AUR package exfat-dkms-git because that package has literally been broken for several months (at least Sept) and I see no activity in the upstream. So all users of that package are broken on kernels post 4.18(.1?). I've been manually using your branch since then.