Add missing tag-prefix to versioneer setup

This is required for versioneer to work with the tag scheme used for
exdir, which is prefixed with a "v".

Author: dragly

setup.cfg

@@ -8,5 +8,5 @@ VCS = git
 style = pep440
 versionfile_source = exdir/_version.py
 versionfile_build = exdir/_version.py
-tag_prefix =
+tag_prefix = v
 parentdir_prefix =