gtk.cfg: keep reporting a bool as the g_ascii_formatd() value

Review of the previous commit: dropping <arg nr="4"> also dropped its
<not-bool/>, so with a format string that is not a literal a bool passed as
the value was no longer reported (invalidFunctionArgBool). Describing arg 4
again makes the format check count it as a fixed argument and report
"requires 1 parameter but only 0 are given" again, so describe it as the
variadic argument instead, as gnu.cfg does.

Author: pduchi

cfg/gtk.cfg

@@ -3298,6 +3298,9 @@
       <strz/>
       <formatstr/>
     </arg>
+    <arg nr="variadic" direction="in">
+      <not-bool/>
+    </arg>
   </function>
   <!-- gboolean g_ascii_isalnum(gchar c); -->
   <function name="g_ascii_isalnum">

test/cfg/gtk.c

@@ -672,3 +672,11 @@ void g_ascii_formatd_test(double d)
     g_ascii_formatd(small, sizeof(buf), "%.2f", d);
     g_print("%s", small);
 }
+
+void g_ascii_formatd_format_test(const gchar *format, int i)
+{
+    gchar buf[39];
+    // cppcheck-suppress invalidFunctionArgBool
+    g_ascii_formatd(buf, sizeof(buf), format, i == 1);
+    g_print("%s", buf);
+}