[gimp] goat-exercises: fix vala build problem

[Jehan <jehanp src gnome org>]

commit 5ad5f67ad09d86704aff250846ce28bdae5755b7
Author: Lukas Oberhuber <lukaso gmail com>
Date:   Tue Feb 8 15:22:55 2022 +0000

    goat-exercises: fix vala build problem
    
    Problem appeared on mac and also on debian unstable. This is autotools
    only as `--disable-since-check` is present
    in meson.build.
    
    Fix suggested by @nielsdg.
    
    ```
    make[3]: Entering directory '/Users/distiller/.cache/jhbuild/build/gimp/extensions/goat-exercises'
      CC       goat-exercise-c.o
    /Users/distiller/gtk/inst/bin/intltool-merge /Users/distiller/gtk/source/gimp/po-plug-ins 
org.gimp.extension.goat-exercises.metainfo.xml.in org.gimp.extension.goat-exercises.metainfo.xml -x -u -c 
../../po-plug-ins/.intltool-merge-cache
      VALAC    goat_exercise_vala_vala.stamp
    Generating and caching the translation database
      CCLD     goat-exercise-c
    /Users/distiller/gtk/source/gimp/extensions/goat-exercises/goat-exercise-vala.vala:28.10-28.18: error: 
`Gimp.main' is not available in gimp-3.0 2.99.9. Use gimp-3.0 >= 3.0
      return Gimp.main(typeof(Goat), args);
             ^^^^^^^^^
    ```

 extensions/goat-exercises/Makefile.am | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)
---
diff --git a/extensions/goat-exercises/Makefile.am b/extensions/goat-exercises/Makefile.am
index 50c44d44cd..44dde13194 100644
--- a/extensions/goat-exercises/Makefile.am
+++ b/extensions/goat-exercises/Makefile.am
@@ -116,7 +116,8 @@ goat_exercise_vala_LDADD = \
 goat_exercise_vala_VALAFLAGS = \
        --vapidir=$(abs_top_builddir)/libgimp   \
        --pkg gimp-@GIMP_API_VERSION@           \
-       --pkg gimp-ui-@GIMP_API_VERSION@
+       --pkg gimp-ui-@GIMP_API_VERSION@        \
+       --disable-since-check
 
 endif