assimp: fix compile for big endian target

Patch taken from upstream [1].

Fixes ([2]):
  code/Bitmap.cpp: In function 'std::size_t Assimp::Copy(uint8_t*, T&) [with T = short unsigned int, std::size_t = unsigned int, uint8_t = unsigned char]':
  code/Bitmap.cpp:95:50:   instantiated from here
  code/Bitmap.cpp:87:9: error: lvalue required as unary '&' operand

[1] https://github.com/assimp/assimp/commit/756cfd4f74b866e3183caede69daa8c105b73bab.patch
[2] http://autobuild.buildroot.net/results/7aa/7aafdc2633bad96a2a17f4e8664e09aae78a3bbd



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