package/assimp: fix musl zlib/zip related compile failure

As assimp fails to use system provided zlib/zip (and is picky
about the provided versions) use assimp contributed one
and backport one part of upstream patch to fix musl compile.

Fixes:

  http://autobuild.buildroot.net/results/4b373ae7d8fd40efe3e2592f94f2d769d6a77669



  In file included from .../assimp-5.0.1/code/3MF/D3MFExporter.cpp:61:
  .../assimp-5.0.1/contrib/zip/src/zip.h:30:15: error: conflicting declaration 'typedef long int ssize_t'
   typedef long  ssize_t;  /* byte count or error */
                 ^~~~~~~

Signed-off-by: Peter Seiderer <ps.report@gmx.net>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>