Using ".dep" for the suffix on header dependency files in the obj directory.

Formerly I used ".i", but "alextingle" on HN observed that that's the suffix
for pre-processed C source.  That shouldn't pose any actual problem, since I
don't expect any other process to generate .i files in the obj directory, but
nevertheless I'm changing the suffix to ".dep" anyway to avoid even the
appearance of any potential problem.