[balsa/wip/gtk4] configure.ac: Fix the build

[Peter Bloomfield <peterb src gnome org>]

commit f60ce00aaff4d5dd6fa93a72f34d4351661323d9
Author: Peter Bloomfield <PeterBloomfield bellsouth net>
Date:   Mon May 28 18:15:41 2018 -0400

    configure.ac: Fix the build
    
    by bumping balsa-ab's Gtk+ version to 4.0, of course!

 configure.ac | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
---
diff --git a/configure.ac b/configure.ac
index 9d9b63ee4..27367df45 100644
--- a/configure.ac
+++ b/configure.ac
@@ -247,7 +247,7 @@ gnutls
 
 PKG_CHECK_MODULES(BALSA_AB, [
    glib-2.0
-   gtk+-3.0
+   gtk+-4.0
    gmime-2.6
 ])