libkmod-module: fill builtin's name

modules.builtin don't have any realname->value -- it follows the same
format of modules.dep, not the aliases ones.
diff --git a/libkmod/libkmod-index.c b/libkmod/libkmod-index.c
index 2a5a26e..be4a2c2 100644
--- a/libkmod/libkmod-index.c
+++ b/libkmod/libkmod-index.c
@@ -485,6 +485,7 @@
  */
 char *index_search(struct index_file *in, const char *key)
 {
+// FIXME: return value by reference instead of strdup
 	struct index_node_f *root;
 	char *value;
 
@@ -958,6 +959,7 @@
  */
 char *index_mm_search(struct index_mm *idx, const char *key)
 {
+// FIXME: return value by reference instead of strdup
 	struct index_mm_node *root;
 	char *value;