[gnome-system-monitor] Added translator commment for cache string

[Robert Roth <robertroth src gnome org>]

commit be60b6519d74bc101b2f284a48587681b3409b32
Author: Robert Roth <robert roth off gmail com>
Date:   Sat Feb 8 09:06:42 2020 +0200

    Added translator commment for cache string

 src/load-graph.cpp | 1 +
 1 file changed, 1 insertion(+)
---
diff --git a/src/load-graph.cpp b/src/load-graph.cpp
index 74fe56ba..153bc5a7 100644
--- a/src/load-graph.cpp
+++ b/src/load-graph.cpp
@@ -418,6 +418,7 @@ namespace
             text = g_strdup_printf(_("%s (%.1f%%) of %s"), used_text, 100.0 * percent, total_text);
 
             if (cached != 0) {
+                // xgettext: Used cache string, e.g.: "Cache: 2.4GiB"
                 cached_label = g_strdup_printf(_("Cache %s"), cached_text);
                 text = g_strdup_printf("%s\n%s", text, cached_label);
                 g_free (cached_label);