gencat/libs.[ch]: keep back pointer to lib in "struct file"

gencat/libs.c

@@ -70,7 +70,8 @@ void add_lib(struct lib *lib, const char *path)
 	file = alloc_type(struct file);
 	file->path = stralloc(path);
 	file->entries = NULL;
-        file->next = lib->files;
+	file->lib = lib;
+	file->next = lib->files;
 	lib->files = file;
 
 	lib->add_lib(lib, path);

gencat/libs.h

@@ -27,6 +27,7 @@ struct entry {
 struct file {
 	const char *path;
 	struct entry *entries;
+	const struct lib *lib;
 	struct file *next;
 };