flashprog: Apply patch for fixing compiler warnings

Quoting the original commit message:

Some toolchains complain that this shadows the global from <getopt.h>.
Alas,  toolchains seem inconsistent in the decision when to warn about
shadowing of globals from system headers and when not to.  Hence, just
rename `optarg' -> `opt_arg'.

Fixes compiler warnings on Darwin and enables building.

Signed-off-by: Felix Singer <felixsinger@posteo.net>

pkgs/by-name/fl/flashprog/package.nix

@@ -1,5 +1,6 @@
 {
   fetchgit,
+  fetchpatch,
   lib,
   libftdi1,
   libgpiod,
@@ -24,6 +25,15 @@ stdenv.mkDerivation (finalAttrs: {
     hash = "sha256-S+UKDtpKYenwm+zR+Bg8HHxb2Jr7mFHAVCZdZTqCyRQ=";
   };
 
+  patches = [
+    # fixes compiler warnings on Darwin
+    (fetchpatch {
+      url = "https://review.sourcearcade.org/changes/flashprog~309/revisions/2/patch?download";
+      hash = "sha256-eiEenR8+CHCJcNx9YY09I7gxRGUQWmaQlmXtykvXyMU=";
+      decode = "base64 -d";
+    })
+  ];
+
   nativeBuildInputs = [
     meson
     ninja