Add missing file in zon file (#94)

The build system requires the `tools/gennodetags/main.zig` file, but
since it's not included in the `build.zig.zon` file, anyone who adds
`pgzx` as a dependency via `zig fetch` (or by specifying the url) will
not be able to build their extension. It will fail with the following
error: `error: unable to load
'/home/guacs/.cache/zig/p/122078a0d7c7c1fc0cee07a61e8101509799873e1f46da165084cd9897f59303652b/tools/gennodetags/main.zig':
FileNotFound`

As a workaround, copying the `tools` directory manually to the directory
containing the source code that `zig` internally creates will work.

Author: vkcku

build.zig.zon

@@ -4,5 +4,6 @@
     .paths = .{
         "build.zig",
         "src",
+        "tools/gennodetags/main.zig",
     },
 }