assimp: added patch to fix hardcoded non-existing paths in CMake modules

assimp CMake modules were adding non-existing paths to its CMake modules
breaking builds for users of assimp. Remove the hardcoded paths with
an upstream patch with small tweaks.

Signed-off-by: Purushottam Choudhary <purushottamchoudhary29@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>