[gtksourceview] build: fix configure on macOS

[Christian Hergert <chergert src gnome org>]

commit dfa2996c7178b7bfdbbe570aac1ecf91335a5c55
Author: Christian Hergert <christian hergert me>
Date:   Wed Dec 22 12:48:12 2021 -0800

    build: fix configure on macOS

 meson.build | 1 +
 1 file changed, 1 insertion(+)
---
diff --git a/meson.build b/meson.build
index 795a3626..23983eb7 100644
--- a/meson.build
+++ b/meson.build
@@ -8,6 +8,7 @@ project('gtksourceview', 'c',
 )
 
 buildtype = get_option('buildtype')
+host_system = host_machine.system()
 
 i18n = import('i18n')
 gnome = import('gnome')