checkpatch: discourage use of g_clear_pointer()

We have nm_clear_pointer() instead, which does not cast the function
argument, and thus the compiler is better at checking the arguments.

contrib/scripts/checkpatch.pl

@@ -193,6 +193,7 @@ complain ("Use gs_free_error with GError variables") if $line =~ /\bgs_free\b +G
 complain ("Don't use strcmp/g_strcmp0 unless you need to sort. Consider nm_streq()/nm_streq0(),NM_IN_STRSET() for testing equality") if $line =~ /\b(strcmp|g_strcmp0)\b/;
 complain ("Don't use API that uses the numeric source id. Instead, use GSource and API like nm_g_idle_add(), nm_g_idle_add_source(), nm_clear_g_source_inst(), etc.") if $line =~ /\b(g_idle_add|g_idle_add_full|g_timeout_add|g_timeout_add_seconds|g_source_remove|nm_clear_g_source)\b/;
 complain ("Prefer g_snprintf() over snprintf() (for consistency)") if $line =~ /\b(snprintf)\b/;
+complain ("Avoid g_clear_pointer() and use nm_clear_pointer() (or nm_clear_g_free(), g_clear_object(), etc.)") if $line =~ /\b(g_clear_pointer)\b/;
 #complain ("Use spaces instead of tabs") if $line =~ /\t/;
 
 # Further on we process stuff without comments.