[gnome-commander/ConvertWarningsToErrors] fix warning: HAVE_GSF will be undefined when unset

[Uwe Scholz <uwescholz src gnome org>]

commit a5598c5e630a980e2fcb6105f4e6e585f148e40c
Author: Andreas Henriksson <andreas fatal se>
Date:   Tue Apr 18 23:37:27 2017 +0200

    fix warning: HAVE_GSF will be undefined when unset
    
    Check if it's defined rather than it's potential value.
    (It will be either defined to 1 or undefined.)

 src/tags/gnome-cmd-tags-doc.cc |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)
---
diff --git a/src/tags/gnome-cmd-tags-doc.cc b/src/tags/gnome-cmd-tags-doc.cc
index 620285b..70a5836 100644
--- a/src/tags/gnome-cmd-tags-doc.cc
+++ b/src/tags/gnome-cmd-tags-doc.cc
@@ -41,7 +41,7 @@
 
 #ifdef HAVE_GSF_1_14_26
 #include <gsf/gsf.h>
-#elif HAVE_GSF
+#elif defined(HAVE_GSF)
 #include <gsf/gsf-infile.h>
 #include <gsf/gsf-infile-msole.h>
 #include <gsf/gsf-infile-zip.h>