travis: also install 32-bit libstdc++
travis: also install 32-bit libstdc++

The build is still broken with missing C++ headers.
Let's see if this helps.

zoulasc/syzkaller,zoulasc/syzkaller,dotweiba/syzkaller,xairy/syzkaller,xairy/syzkaller,dotweiba/syzkaller,xairy/syzkaller,google/syzkaller,google/syzkaller,zoulasc/syzkaller,dotweiba/syzkaller,google/syzkaller,zoulasc/syzkaller,xairy/syzkaller,google/syzkaller,xairy/syzkaller,dotweiba/syzkaller,google/syzkaller,google/syzkaller,google/syzkaller,xairy/syzkaller,xairy/syzkaller