imlib2, lightmediascanner: Add PRIVATE_LIBS

imlib2, lightmediascanner: Add PRIVATE_LIBS

* to resolve following warning:
  WARNING: lightmediascanner-0.4.4 is already registered as shlib provider for png.so, ignoring imlib2-loaders-1.4.5+svnr82070 trying to register the same
  WARNING: lightmediascanner-0.4.4 is already registered as shlib provider for jpeg.so, ignoring imlib2-loaders-1.4.5+svnr82070 trying to register the same
  WARNING: lightmediascanner-0.4.4 is already registered as shlib provider for id3.so, ignoring imlib2-loaders-1.4.5+svnr82070 trying to register the same
  when fixes from https://bugzilla.yoctoproject.org/show_bug.cgi?id=4628
  are included in build

(From meta-openembedded rev: c11f208cb2d00666c84a0d07644c4bf8ac5adf3c)

Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>
Signed-off-by: Patrick Ohly <patrick.ohly@intel.com>