Workaround build failure on -fno-common toolchains like upstream
gcc-10. Otherwise build fails as:
ld: modules/.libs/libmodules.a(rocket_pad.o):/build/lincity-1.13.1/modules/../screen.h:23:
multiple definition of `monthgraph_style'; ldsvguts.o:/build/lincity-1.13.1/screen.h:23: first defined here