Fix warnings in system/ headers

system/core/include is included in the global include path using
-isystem, which hides all warnings.  Fix warnings in
system/core/include/system in preparation for moving from -isystem to -I.

- Fix invalid doxygen syntax in graphics.h
- Use a pragma to hide a C99 extension warning on the flexible length
  array in graphics.h
- Make static functions in radio.h static inline
- Fix size_t printf and size_t vs. ssize_t comparision in qemu_pipe.h
- Fix old style cast in window.h

Test: m -j native
Bug: 31492149
Change-Id: I857f289e4c8e303494831873282bbb69de155c10