Fix compilation with gcc10 / -fno-common
gcc10 defaults to -fno-common, meaning that non static symbols can only be declared once. Make the parent_class variable in level.c and theme.c static to fix building with gcc10.
gcc10 defaults to -fno-common, meaning that non static symbols can only be declared once. Make the parent_class variable in level.c and theme.c static to fix building with gcc10.