buildroot

gcc: add missing NIOS-II patch after bump to 4.9.3

When gcc 4.9.x was bumped from 4.9.2 to 4.9.3 in commit
2fed00ea1eaeca952f872120af79a46f39b0868c, patch
920-libgcc-remove-unistd-header.patch was removed with the argument
that it had been applied upstream.

However, it is not the case, and the patch continues to apply fine on
gcc 4.9.3, and is actually needed to make gcc build properly on
NIOS-II.

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