postgresql18Packages.pg_hll: fix on darwin

Disabling the `missing-variable-declarations` warning only was part of
the solution, it didn't fix darwin builds. Applying upstream's patch
fixes it on all platforms. It will also warn us via build failure when
this isn't needed anymore. Win-win!

pkgs/servers/sql/postgresql/ext/pg_hll.nix

@@ -1,5 +1,6 @@
 {
   fetchFromGitHub,
+  fetchpatch,
   lib,
   postgresql,
   postgresqlBuildExtension,
@@ -16,8 +17,13 @@ postgresqlBuildExtension (finalAttrs: {
     hash = "sha256-Latdxph1Ura8yKEokEjalJ+/GY+pAKOT3GXjuLprj6c=";
   };
 
-  # https://github.com/citusdata/postgresql-hll/issues/166#issuecomment-3165489050
-  NIX_CFLAGS_COMPILE = "-Wno-error=missing-variable-declarations";
+  patches = [
+    (fetchpatch {
+      name = "fix-postgresql-18.patch";
+      url = "https://github.com/citusdata/postgresql-hll/commit/f998e234653ea397ddddc1278d1c02d8d011bd16.patch";
+      hash = "sha256-gF4f+B4Gu/QEyCGMfKLmRK6lNwgfd8lML55wMkhsSY4=";
+    })
+  ];
 
   meta = {
     description = "HyperLogLog for PostgreSQL";