[libgda] Meson: Fix build documentation without UI

[Daniel Espinosa Ortiz <despinosa src gnome org>]

commit 6785af4e7f758551b9facc124bf4882a6b37ca51
Author: Daniel Espinosa <esodan gmail com>
Date:   Thu Apr 4 17:39:11 2019 -0600

    Meson: Fix build documentation without UI

 doc/C/meson.build | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)
---
diff --git a/doc/C/meson.build b/doc/C/meson.build
index d4e57e74f..a9ad5ed3c 100644
--- a/doc/C/meson.build
+++ b/doc/C/meson.build
@@ -1,2 +1,4 @@
 subdir('libgda')
-subdir('libgdaui')
\ No newline at end of file
+if enable_ui
+subdir('libgdaui')
+endif