Build fails on gcc-10 (-fno-common changes)

[trofi]

gcc-10 changed the default from -fcommon to fno-common:
https://gcc.gnu.org/PR85678

As a result build fails as:

   ld: sprite.o:/build/0verkill/data.h:171: multiple definition of `obj_attr'; server.o:/build/0verkill/data.h:171: first defined here
   ld: sprite.o:/build/0verkill/data.h:142: multiple definition of `weapon_name'; server.o:/build/0verkill/data.h:142: first defined here
   ld: hash.o:/build/0verkill/data.h:187: multiple definition of `weapon'; server.o:/build/0verkill/data.h:187: first defined here
   ld: hash.o:/build/0verkill/data.h:171: multiple definition of `obj_attr'; server.o:/build/0verkill/data.h:171: first defined here
   ld: hash.o:/build/0verkill/data.h:142: multiple definition of `weapon_name'; server.o:/build/0verkill/data.h:142: first defined here
   ld: error.o:/build/0verkill/data.h:187: multiple definition of `weapon'; server.o:/build/0verkill/data.h:187: first defined here
   ld: error.o:/build/0verkill/data.h:171: multiple definition of `obj_attr'; server.o:/build/0verkill/data.h:171: first defined here
   ld: error.o:/build/0verkill/data.h:142: multiple definition of `weapon_name'; server.o:/build/0verkill/data.h:142: first defined here

The fix would normally to leave only extern declaration of the variable in header files and move definition to .c files.