meson: Use gnome.mkenum_simple()

Meson now uses find_program() to get glib-mkenum from glib instead of
from system. That was already fixed at least in >=0.60 which is our
current minimum requirement.

meson.build

@@ -9,6 +9,8 @@ project('glib', 'c',
   ]
 )
 
+gnome = import('gnome')
+
 cc = meson.get_compiler('c')
 c_standards = {}