python-pip: add python-distribute in RDEPENDS

Fix the following runtime issue of pip:
    # pip
    Traceback (most recent call last):
      File "/usr/bin/pip", line 5, in <module>
        from pkg_resources import load_entry_point
    ImportError: No module named pkg_resources

Signed-off-by: Zhenhua Luo <zhenhua.luo@freescale.com>