meson: Update syn subproject

Fixes the following build error for fedora-release.

Error: method `inner` is never used
   --> ../subprojects/syn-2.0.39/src/attr.rs:589:8
    |
585 | pub(crate) trait FilterAttrs<'a> {
    |                  ----------- method in this trait
...
589 |     fn inner(self) -> Self::Ret;
    |        ^^^^^
    |
    = note: `-D dead-code` implied by `-D warnings`
    = help: to override `-D warnings` add `#[allow(dead_code)]`
error: aborting due to 1 previous error

Signed-off-by: Christian Gmeiner <cgmeiner@igalia.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/29996>

subprojects/syn.wrap

@@ -1,6 +1,6 @@
 [wrap-file]
-directory = syn-2.0.39
-source_url = https://crates.io/api/v1/crates/syn/2.0.39/download
-source_filename = syn-2.0.39.tar.gz
-source_hash = 23e78b90f2fcf45d3e842032ce32e3f2d1545ba6636271dcbf24fa306d87be7a
+directory = syn-2.0.68
+source_url = https://crates.io/api/v1/crates/syn/2.0.68/download
+source_filename = syn-2.0.68.tar.gz
+source_hash = 901fa70d88b9d6c98022e23b4136f9f3e54e4662c3bc1bd1d84a42a9a0f0c1e9
 patch_directory = syn