Add -Werror, warnings is not a good thing to have.

Warnings mostly appears by a reason, and is almost always just easy
to fix. Therefore, treat warnings as errors, so we find all those
"implicit declartation" and dangerous implict typecasts.

Also fix the warnings that appeared

Change-Id: I7012964838c76127d3d45dabc533c344b12f5bd5
Signed-off-by: Max Sikstrom <msikstrom@op5.com>