[shotwell] Fix running video thumbnailer from build dir

[Jens Georg <jensgeorg src gnome org>]

commit 668b7a4cca14dcc4079748e45bb5507833fc4f4e
Author: Jens Georg <mail+gitkraken jensge org>
Date:   Sat Mar 31 21:48:42 2018 +0200

    Fix running video thumbnailer from build dir

 src/AppDirs.vala |    5 +++++
 1 files changed, 5 insertions(+), 0 deletions(-)
---
diff --git a/src/AppDirs.vala b/src/AppDirs.vala
index ffea859..d338909 100644
--- a/src/AppDirs.vala
+++ b/src/AppDirs.vala
@@ -302,6 +302,11 @@ class AppDirs {
             // If we're running installed.
             f = AppDirs.get_libexec_dir () .get_child ("shotwell").get_child (filename);
         }
+
+        if (!f.query_exists()) {
+            f = AppDirs.get_libexec_dir().get_parent().get_child("thumbnailer").get_child(filename);
+        }
+
         return f;
     }