[shotwell] Prevent compile issue with VALA < 0.40

[Jens Georg <jensgeorg src gnome org>]

commit 56e89a319b93fa29526fbcf8bae217056fe7c7d8
Author: Jens Georg <mail jensge org>
Date:   Sun Apr 21 15:02:55 2019 +0200

    Prevent compile issue with VALA < 0.40

 src/VideoSupport.vala | 4 ++++
 1 file changed, 4 insertions(+)
---
diff --git a/src/VideoSupport.vala b/src/VideoSupport.vala
index f6371c40..533dfe67 100644
--- a/src/VideoSupport.vala
+++ b/src/VideoSupport.vala
@@ -232,7 +232,11 @@ public class VideoReader {
         debug("Thumbnailer timer called");
         if (thumbnailer_pid != 0) {
             debug("Killing thumbnailer process: %d", thumbnailer_pid);
+#if VALA_0_40
             Posix.kill(thumbnailer_pid, Posix.Signal.KILL);
+#else
+            Posix.kill(thumbnailer_pid, Posix.SIGKILL);
+#endif
         }
         return false; // Don't call again.
     }