pkg-config is unable to autodetect whether a package is linked
statically or not. This helper just moves Libs.private into Libs
manually in case only a static library is built. Most if not all of
contribs are built that way.
(cherry picked from commit 3ce29fe10e2e9813452abbbd1a0aaaea07a57411)
Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>