Update min version of gettext required to build with .desktop files

.desktop files support was not added to msgfmt until v0.19
diff --git a/vncviewer/CMakeLists.txt b/vncviewer/CMakeLists.txt
index e04e335..2aecda2 100644
--- a/vncviewer/CMakeLists.txt
+++ b/vncviewer/CMakeLists.txt
@@ -62,7 +62,7 @@
 
   configure_file(vncviewer.desktop.in.in vncviewer.desktop.in)
   find_program(INTLTOOL_MERGE_EXECUTABLE intltool-merge)
-  if("${GETTEXT_VERSION_STRING}" VERSION_GREATER 0.17.99)
+  if("${GETTEXT_VERSION_STRING}" VERSION_GREATER 0.18.99)
     add_custom_command(OUTPUT vncviewer.desktop
       COMMAND ${GETTEXT_MSGFMT_EXECUTABLE}
                 --desktop --template vncviewer.desktop.in