[libsoup] meson: switch documentation option from 'doc' to 'gtk_doc'

[Claudio Saavedra <csaavedra src gnome org>]

commit 800bc6b8ab7a59676ca21f066ddf1d7a1326d409
Author: Claudio Saavedra <csaavedra igalia com>
Date:   Thu May 16 16:22:58 2019 +0300

    meson: switch documentation option from 'doc' to 'gtk_doc'
    
    This is the standard in GNOME currently.

 meson.build | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
---
diff --git a/meson.build b/meson.build
index 89837c15..a212781b 100644
--- a/meson.build
+++ b/meson.build
@@ -321,6 +321,6 @@ if get_option('tests')
   subdir('tests')
  endif
 
-if get_option('doc')
+if get_option('gtk_doc')
   subdir('docs/reference')
 endif