Previously, the location of `GIT_WARN_UNUSED_RESULT` was causing it to
be included _after_ a bunch of other headers (namely `src/vector.h`),
which broke the build.

This change does two things:

* Moves the `GIT_WARN_UNUSED_RESULT` above most of the `$include`s in
  `src/common.h`.
* Stops including `vector.h` from `src/win32/path_w32.c` since the
  header itself does not use it.