assimp: fix gcc version condition

In commit 68cebede ("assimp: work around
gcc bug on SuperH"), a work around was added to make the package build
with gcc on SuperH. The condition included a test on the gcc version,
which was mistakenly done on the host gcc version, while a test on the
target gcc version was intended.

Thanks to Peter Korsgaard for spotting the issue.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>