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.

pkgs/development/tools/libsigrok/default.nix

···
     sha256 = "0l3h7zvn3w4c1b9dgvl3hirc4aj1csfkgbk87jkpl7bgl03nk4j3";
   };
 
+  firmware = fetchurl {
+    url = "http://sigrok.org/download/binary/sigrok-firmware-fx2lafw/sigrok-firmware-fx2lafw-bin-0.1.2.tar.gz";
+    sha256 = "0w0w6l015d16181mx8mgyjha4bv3ba7x36p86k9n1x52809433gj";
+  };
+
   buildInputs = [ pkgconfig libzip glib libusb1 libftdi check libserialport
     librevisa
   ];
+
+  postInstall = ''
+    mkdir -p "$out/share/sigrok-firmware/"
+    tar --strip-components=1 -xvf "${firmware}" -C "$out/share/sigrok-firmware/"
+  '';
 
   meta = with stdenv.lib; {
     description = "Core library of the sigrok signal analysis software suite";