pahole: Add support for finding pointers to void

  $ pahole --find_pointers_to void --prefix tcp
  tcp_md5sig_pool: scratch
  $ pahole tcp_md5sig_pool
  struct tcp_md5sig_pool {
  	struct ahash_request *     md5_req;              /*     0     8 */
  	void *                     scratch;              /*     8     8 */

  	/* size: 16, cachelines: 1, members: 2 */
  	/* last cacheline: 16 bytes */
  };
  $

Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>
diff --git a/pahole.c b/pahole.c
index 7d5229d..829c9ab 100644
--- a/pahole.c
+++ b/pahole.c
@@ -1226,8 +1226,11 @@
 		struct tag *class = cu__find_type_by_name(cu, pos->s, include_decls, &class_id);
 		if (class == NULL) {
 			class = cu__find_base_type_by_name(cu, pos->s, &class_id);
-			if (class == NULL)
-				continue;
+			if (class == NULL) {
+				if (strcmp(pos->s, "void"))
+					continue;
+				class_id = 0;
+			}
 		}
 
 		if (defined_in) {
@@ -1240,15 +1243,16 @@
 		 */
 		strlist__remove(class_names, pos);
 
-		class__find_holes(tag__class(class));
+		if (class)
+			class__find_holes(tag__class(class));
 		if (reorganize) {
-			if (tag__is_struct(class))
+			if (class && tag__is_struct(class))
 				do_reorg(class, cu);
 		} else if (find_containers)
 			print_containers(cu, class_id, 0);
 		else if (find_pointers_in_structs)
 			print_structs_with_pointer_to(cu, class_id);
-		else {
+		else if (class) {
 			/*
 			 * We don't need to print it for every compile unit
 			 * but the previous options need