timezonespec: move to an implementation file

If the header is included in multiple TUs, the symbol ends up existing
in each one. This causes duplicate symbols if the TUs ever meet.
13 jobs for timezonespec-duplicate-symbol
Status Job ID Name Coverage
  External
passed cdash

passed ghostflow-branch-check-master

failed smtk@master-kerbin-windows-shared-release+examples+remus

00:07:11

passed smtk@master-moho-linux-shared-relwithdebinfo+examples+private+remus

00:10:53

failed smtk@master-moho-linux-shared-relwithdebinfo+examples+python3+remus+vtk

00:00:06

passed smtk@master-moho-linux-shared-relwithdebinfo+examples+remus

00:11:45

passed smtk@master-moho-linux-shared-relwithdebinfo+examples+remus+vtk

00:08:30

passed smtk@master-norkit-osx-shared-release+examples+matplotlib

00:26:36

running smtk@master-praxis-windows-shared-release

639:56:48

passed smtk@master-talosiv-osx-shared-release+examples+matplotlib

00:13:14

passed smtk@master-tatooine-osx-shared-release+examples+matplotlib

00:11:07

failed smtk@master-tatooine-osx-shared-release+examples+matplotlib+private

00:11:15

passed cdash