Add new version macros.

* src/gpg-error.h.in (GPGRT_VERSION): New.
(GPGRT_VERSION_NUMBER): New.
(GPG_ERROR_VERSION, GPG_ERROR_VERSION_NUMBER): Move to top of file.

Eventually we will have a gpgrt.h file instead of gpg-error. To make
things easier we already provide suitable named version macros.
Miving them to the top makes it easier to find the version.

• Signed-off-by: Werner Koch <wk@gnupg.org>