Make built extensions depend on golang/{sync,time}.pxd and rebuild if
those files change. A bit overkill to depend globally, but more correct,
since an extension that is using pygolang can be using those packages.

Before the patch - without those depends - since distutils does not
implement proper dependency tracking, extensions - even those that
actually depend on sync/time pxd, were not rebuilt.

sync.pxd was missed to be added in 34b7a1f4 (golang: Expose Sema and
Mutex as public Python and Cython/nogil API). time.pxd was missed to be
added in ce8152a2 (pyx api: Provide sleep).