kmscube: pull upstream fix for -fno-common toolchains
Without the change build against upstream gcc-10 fails as:
ld: kmscube-cube-tex.o:/build/kmscube-9dcce71/cube-tex.c:48: multiple definition of `gl';
kmscube-cube-smooth.o:/build/kmscube-9dcce71/cube-smooth.c:40: first defined here