Adjust src-release.sh's getver due to gdbsupport's move to toplevel

Adjust src-release.sh's getver due to gdbsupport's move to toplevel

The move of gdbsupport to the top level directory requires a small
change to src-release.sh's "getver" function, which is responsible for
determining the version string that will be appended to the release
tarball: now the create-version.sh script lives under ./gdbsupport,
and not under gdb/gdbsupport anymore.

This patch unbreaks the snapshot generation, which hasn't been working
since January 14th.

ChangeLog:
2020-01-29  Sergio Durigan Junior  <sergiodj@redhat.com>

	* src-release.sh (getver): Look for gdbsupport's
	create-version.sh script at the current directory if tool is
	"gdb".

Change-Id: Id3b8bed6583a1aaa120c07009366f6c94a62d5db