Avoid race condition when building outside of source tree

Avoid race condition when building outside of source tree

When building outside of source tree it can happen that src/ostree/
does not exist (yet) when bison is called. This leads to an build
error like so:
  bison: src/ostree/parse-datetime.c: cannot open: No such file or directory

Make sure that src/ostree/ exists when parse-datetime.c is built.