[gtkmm] CI: Select latest released version of libsigc++3

[Kjell Ahlstedt <kjellahl src gnome org>]

commit e9b34d913e89236f0719a24471de3332832502cb
Author: Kjell Ahlstedt <kjellahlstedt gmail com>
Date:   Sun Feb 27 09:48:55 2022 +0100

    CI: Select latest released version of libsigc++3

 .gitlab-ci.yml | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)
---
diff --git a/.gitlab-ci.yml b/.gitlab-ci.yml
index e2aaa2d4..634c97fa 100644
--- a/.gitlab-ci.yml
+++ b/.gitlab-ci.yml
@@ -1,4 +1,4 @@
-# 2022-02-08: ubuntu:latest = 20.04, ubuntu:rolling = 21.10, ubuntu:devel = 22.04
+# 2022-02-27: ubuntu:latest = 20.04, ubuntu:rolling = 21.10, ubuntu:devel = 22.04
 # See https://hub.docker.com/_/ubuntu
 image: ubuntu:devel
 
@@ -63,7 +63,7 @@ build_deps:
     # Build and cache dependencies that can't be installed with apt.
     # Build libsigc++3
     - if test ! -d libsigc; then
-    - git clone --branch 3.0.7 --depth 1 https://github.com/libsigcplusplus/libsigcplusplus.git libsigc
+    - git clone --branch 3.2.0 --depth 1 https://github.com/libsigcplusplus/libsigcplusplus.git libsigc
     - cd libsigc
     - mkdir _build && cd _build
     - meson --prefix=/usr --libdir=lib -Dbuild-documentation=false -Dbuild-examples=false -Dbuildtype=release