gtkplus: package cairo dependency must be +gobject (#12655)

e891cc4 updated the cairo package to make gobject support optional
and disabled it by default. gtkplus requires gobject support so this
explicitly enables +gobject on the cairo dependency of gtkplus.

var/spack/repos/builtin/packages/gtkplus/package.py

@@ -25,7 +25,7 @@ class Gtkplus(AutotoolsPackage):
     # Hardcode X11 support (former +X variant),
     # see #6940 for rationale:
     depends_on('pango+X')
-    depends_on('cairo+X+pdf')
+    depends_on('cairo+X+pdf+gobject')
     depends_on('gobject-introspection')
     depends_on('libepoxy', when='@3:')
     depends_on('libxi', when='@3:')