Run through IWYU and fix issues
This is picked up from: #219 (comment 148197)
Run Weston sources through include-what-you-use and at least remove all includes that are completely unused.
Weston does not follow the expected file naming of IWYU though, so adjusting the tool and consideration will be needed. A header file should not
#include anything it does not itself directly use, but there may be some exceptions in the public installed headers.