djmount: add -fcommon workaround
djmount: add -fcommon workaround

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

    ld: libupnp/upnp/.libs/libupnp.a(libupnp_la-gena_ctrlpt.o):libupnp/upnp/src/inc/upnpapi.h:163:
      multiple definition of `pVirtualDirList'; libupnp/upnp/.libs/libupnp.a(libupnp_la-upnpapi.o):libupnp/upnp/src/inc/upnpapi.h:163: 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