build: Ensure enumtypes.h is built before enumtypes.c
build: Ensure enumtypes.h is built before enumtypes.c

ostree-enumtypes.c includes ostree-enumtypes.h, so make needs to be told
about the dependency. Without it, parallel make could try to build
ostree-enumtypes.c before the header file exists.

I hit this when running `make -j OSTree-1.0.gir`.

Closes: #1322
Approved by: pwithnall

GNOME/ostree,GNOME/ostree,GNOME/ostree,GNOME/ostree,GNOME/ostree