libav: add libpostproc to PROVIDES (for 0.8.x version only)

libav: add libpostproc to PROVIDES (for 0.8.x version only)

There is a separate libpostproc recipe in meta-oe for use with 9.x and
later versions of libav for those few that need libpostproc; however if
you just add meta-oe and try to build libpostproc without selecting the
libav 9.x version recipe, you'll be building the libpostproc recipe
together with libav 0.8.x, which provides its own libpostproc; this
leads to confusing errors at packaging time. In order to flag up that
these conflict more appropriately, add libpostproc to PROVIDES
explicitly so that you at least get a multiple providers error at the
start of the build.

Fixes [YOCTO #5335].

(From OE-Core rev: e8f9420fe901675fc1a8d4e41302c2faa4a7dc4a)

Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>