Fix header inclusion issue.

Fix header inclusion issue.

partprune.h failed to compile by itself; needs to include partdefs.h.

I think I must've broken this in fa2cf164a, though I'd swear I ran
the appropriate tests when removing #includes.  Anyway, it's very
sensible for this file to include partdefs.h, so let's just do that.

Per cpluspluscheck.