Include sys/uio.h

[GregMefford]

When using Picam with more recent Nerves versions, it looks like the gcc defaults are different, making writev no longer available by default. Explicitly including sys/uio.h resolves this.

src/raspijpgs.c: In function 'output_jpeg':
src/raspijpgs.c:727:21: warning: implicit declaration of function 'writev'; did you mean 'write'? [-Wimplicit-function-declaration]
     ssize_t count = writev(STDOUT_FILENO, iovs, 2);
                     ^~~~~~
                     write

I was going to propose that we release this simple change as a 0.3.1 bugfix, but it looks like there's already also something else merged against the 0.4.0-dev branch, so we can call it 0.4.0 instead.

[electricshaman]

Looks good!