oeqa/sdk/assimp: run only when zlib is in the SDK

oeqa/sdk/assimp: run only when zlib is in the SDK

Otherwise assimp will silently fall back to a vendored copy of zlib
which will fail with -D_TIME_BITS=64 due to https://github.com/madler/zlib/pull/764

This was exposed by multilib mips core-image-minimal SDKs, where the default
64 bit sysroot has zlib, but 32 bit sysroot does not.

Signed-off-by: Alexander Kanavin <alex@linutronix.de>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>