packetdrill: pull upstream fix for fno-common toolchains (#141574)

* packetdrill: pull upstream fix for fno-common toolchains

Example toolchain is clang-12. The patch fixes the following
build failure:

$ nix-build -E 'with import ./. {}; packetdrill.override { stdenv = clang12Stdenv; }'
...
ld: libpacketdrill-lib.a(tcp_options_to_string.c.o):(.bss+0x0):
multiple definition of `tcp_option';
libpacketdrill-lib.a(run.c.o):(.bss+0x18): first defined here

pkgs/tools/networking/packetdrill/default.nix

···
-{ lib, stdenv, fetchFromGitHub, bison, flex, cmake, libpcap }:
+{ lib, stdenv, fetchFromGitHub, fetchpatch, bison, flex, cmake, libpcap }:
 stdenv.mkDerivation rec {
   pname = "packetdrill";
   version = "unstable-2020-08-22";
···
     rev = "68a34fa73cf221e5f52d6fa4f203bcd93062be1b";
     sha256 = "0djkwb6l2959f44d98vwb092rghf0qmii8391vrpxqb99j6pv4h6";
   };
+  patches = [
+    # Upstream fix for -fno-common toolchains
+    (fetchpatch {
+      name = "fno-common.patch";
+      url = "https://github.com/google/packetdrill/commit/c08292838de81a71ee477d5bf9d95b1130a1292b.patch";
+      sha256 = "1irbar1zkydmgqb12r3xd80dwj2jfxnxayxpb4nmbma8xm7knb10";
+      stripLen = 3;
+    })
+  ];
+
   setSourceRoot = ''
     export sourceRoot=$(realpath */gtests/net/packetdrill)
   '';