dpkg-gensymbols: skip directories during scan of package build dir

dpkg-gensymbols: skip directories during scan of package build dir

* scripts/dpkg-gensymbols: Don't scan directories which are
symlinks. They might point outside of the package build dir which
results in addition of libraries which are not in the package.
The libc6 package on amd64 is affected with a symlink "lib64" ->
"/lib".