Remove '' in Makefile

[gfvvz]

I think there is a typo in the current Makefile, it should remove '' to make sed comand like this:

sed -i '1s/^/`timescale 1ns/1ns\n/' build/gpu.v

original one is

sed -i '' '1s/^/`timescale 1ns/1ns\n/' build/gpu.v