src-release: Fix VER computation for TOOL=gdb

Without this patch, the DATE marker in gdb/version.in does not get
replaced by the source packaging date, causing the name of the
tarball being created to: have the DATE marker in the tarball
name and the name of the directory the sources unpack to (Eg:
gdb-7.6.50-DATE-cvs.tar instead of gdb-7.6.50-20130710-cvs.tar).

ChangeLog:

        * src-release (VER): Use $(TOOL)/common/create-version.sh
        if it exists.