[libgweather/tests-option] Ignore gi-docgen's subproject

[Emmanuele Bassi <ebassi src gnome org>]

commit c2d1d2f2e49a096801cc852ab73103a5ec9a68ed
Author: Emmanuele Bassi <ebassi gnome org>
Date:   Sat Oct 16 22:00:17 2021 +0100

    Ignore gi-docgen's subproject
    
    If we clone it, we don't really need it to mess with Git.

 subprojects/.gitignore | 1 +
 1 file changed, 1 insertion(+)
---
diff --git a/subprojects/.gitignore b/subprojects/.gitignore
new file mode 100644
index 00000000..3e838ad1
--- /dev/null
+++ b/subprojects/.gitignore
@@ -0,0 +1 @@
+gi-docgen/