libsigrok: install missing firmware files
libsigrok: install missing firmware files

Without this, hardware that requires firmware (e.g. Saleae Logic) is
unuseable.

Example error that this fixes:

  sr: ezusb: Unable to open firmware file /nix/store/rqgppsz3gwy8jailflf3049fzyzrgkdb-libsigrok-0.3.0/share/sigrok-firmware/fx2lafw-saleae-logic.fw for reading: No such file or directory
  sr: fx2lafw: Firmware upload failed for device 0.

NixOS/nixpkgs,SymbiFlow/nixpkgs,SymbiFlow/nixpkgs,triton/triton,NixOS/nixpkgs,NixOS/nixpkgs,triton/triton,SymbiFlow/nixpkgs,triton/triton,triton/triton,SymbiFlow/nixpkgs,SymbiFlow/nixpkgs,SymbiFlow/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,SymbiFlow/nixpkgs,NixOS/nixpkgs,triton/triton,triton/triton,NixOS/nixpkgs,NixOS/nixpkgs,triton/triton,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,SymbiFlow/nixpkgs,SymbiFlow/nixpkgs,NixOS/nixpkgs,SymbiFlow/nixpkgs,SymbiFlow/nixpkgs,triton/triton,SymbiFlow/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,SymbiFlow/nixpkgs