build: Avoid file name length limitation of tar
"csharpdoc" is a bit too long to accommodate long file names with
the version returned from git-version-gen:
tar: gettext-0.19.6.4-1e74e4/.../csharpdoc/...: file name is too
long (max 99); not dumped
* gettext-runtime/intl-csharp/doc: Rename from csharpdoc.
* gettext-runtime/intl-cshap/Makefile.am (doc, DOC_FILES): Adjust to the
directory name change.
