Fix v4l2 wrapper usage for 32 bit systems

[eifert]

Hi, when trying out rscam on Ubuntu 20.04 armhf (Armbian on BananaPI M1), I saw that the build with standard features (i.e. with the v4l wrappers) failed to mmap, while the "no_wrapper" build worked fine (but of course could not do any format conversions). Turns out that mmap and v4l_mmap have a nasty signature mismatch that's not reflected in rscam.

[loyd]

Thank you for your contribution.

Does PR close #9?

[eifert]

Thank you for your contribution.

Does PR close #9?

I can say that I got the same error message as the reporter, and this PR resolves the root cause of my error message. But chances are good that that issue has exactly the same root cause.