mlmmj: add -fcommon workaround
mlmmj: add -fcommon workaround

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

    ld: getlistdelim.o:/build/mlmmj-1.3.0/src/../include/mlmmj.h:84: multiple definition of
      `subtype_strs'; mlmmj-send.o:/build/mlmmj-1.3.0/src/../include/mlmmj.h:84: first defined here

NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs