assimp: build fix for 32-bit architectures

Rather than add to the ifdef stew that's already there, simply don't
try to second-guess the host platform. Otherwise, we'd need more macro
checks added for, at least, NetBSD, FreeBSD, and musl, but including
sys/types.h should "just work".

(There's seemingly another unrelated portability issue directly below
that code block, but it won't break builds, and would require different
means of correction.)

(gutteridge)