skktools: fix build with gcc15

- add patch from merged upstream PR:
https://www.github.com/skk-dev/skktools/pull/30
https://github.com/skk-dev/skktools/commit/fb6a295607dbe2b5171c2c89f8a2f0b82bee9766

Fixes build failure with gcc15:
```
./skkdic-expr.c:359:13: error: too many arguments to function
'add_content_line'; expected 0, have 3
  359 |         if (add_content_line(new, content, NULL))
      |             ^~~~~~~~~~~~~~~~ ~~~
./skkdic-expr.c:115:12: note: declared here
  115 | static int add_content_line();
      |            ^~~~~~~~~~~~~~~~
./skkdic-expr.c:576:9: error: too many arguments to function
'subtract_content_line'; expected 0, have 3
  576 |         subtract_content_line(new, content, NULL);
      |         ^~~~~~~~~~~~~~~~~~~~~ ~~~
./skkdic-expr.c:116:13: note: declared here
  116 | static void subtract_content_line();
      |             ^~~~~~~~~~~~~~~~~~~~~
```

pkgs/by-name/sk/skktools/package.nix

@@ -2,6 +2,7 @@
   lib,
   stdenv,
   fetchFromGitHub,
+  fetchpatch,
   pkg-config,
   gdbm,
   glib,
@@ -34,6 +35,16 @@ stdenv.mkDerivation rec {
   #   sha256 = "1k9zxqybl1l5h0a8px2awc920qrdyp1qls50h3kfrj3g65d08aq2";
   # };
 
+  patches = [
+    # Fix build with gcc15
+    # https://github.com/skk-dev/skktools/pull/30
+    (fetchpatch {
+      name = "skktools-fix-function-prototype-empty-arguments-gcc15.patch";
+      url = "https://github.com/skk-dev/skktools/commit/fb6a295607dbe2b5171c2c89f8a2f0b82bee9766.patch";
+      hash = "sha256-wao2kRsDq5WN4JO/YpXhNirsdnA3vZpsY9GDCTPSJKY=";
+    })
+  ];
+
   nativeBuildInputs = [ pkg-config ];
   buildInputs = [
     gdbm