glib.supp: Suppress reachable leak of g_set_prgname() string

It’s set once and then used throughout the process lifetime.

Signed-off-by: Philip Withnall <withnall@endlessm.com>

Fixes: #2136

glib.supp

@@ -996,3 +996,13 @@
 	...
 	fun:g_thread_pool_new
 }
+
+# One-time program name storage
+{
+	g_set_prgname
+	Memcheck:Leak
+	match-leak-kinds:reachable
+	fun:malloc
+	...
+	fun:g_set_prgname
+}