yersinia: add -fcommon workaround
yersinia: add -fcommon workaround

Workaround build failure on -fno-common toolchains like upstream
gcc-10. Otherwise build fails as:

    ld: ncurses-interface.o:src/ncurses-interface.h:83: multiple definition of
      `pointer'; ncurses-gui.o:src/ncurses-interface.h:83: first defined here

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