[gthumb] Add no-c-format comment for gettext

[Marek Černocký <mcernocky src gnome org>]

commit 9564139c158450cecfd83a8027c440c222185ba1
Author: Marek Černocký <marek manet cz>
Date:   Mon Aug 22 13:42:19 2016 +0200

    Add no-c-format comment for gettext

 .../file_tools/gth-file-tool-adjust-contrast.c     |    1 +
 1 files changed, 1 insertions(+), 0 deletions(-)
---
diff --git a/extensions/file_tools/gth-file-tool-adjust-contrast.c 
b/extensions/file_tools/gth-file-tool-adjust-contrast.c
index 392ba62..f0eb65c 100644
--- a/extensions/file_tools/gth-file-tool-adjust-contrast.c
+++ b/extensions/file_tools/gth-file-tool-adjust-contrast.c
@@ -531,6 +531,7 @@ gth_file_tool_adjust_contrast_get_options (GthFileTool *base)
                                    METHOD_STRETCH_0_5,
                                    get_image_task_for_method (METHOD_STRETCH_0_5),
                                    _("Stretch"),
+                                   /* xgettext:no-c-format */
                                    _("Stretch the histogram after trimming 0.5% from both ends"));
        gth_filter_grid_add_filter (GTH_FILTER_GRID (filter_grid),
                                    METHOD_EQUALIZE_SQUARE_ROOT,