Trace function signature types

This extends the `TypeCollector` trait implementation for `Item` to
consider items of kind `Function` and to collect the types found in the
function's signature.

Fixes rust-lang#291

Author: fitzgen

libbindgen/src/ir/item.rs

@@ -142,6 +142,11 @@ impl TypeCollector for Item {
                     ty.collect_types(ctx, types, self);
                 }
             }
+            ItemKind::Function(ref fun) => {
+                if !self.is_opaque(ctx) {
+                    types.insert(fun.signature());
+                }
+            }
             _ => {} // FIXME.
         }
     }

libbindgen/tests/expectations/tests/type-referenced-by-whitelisted-function.rs

@@ -0,0 +1,7 @@
+/* automatically generated by rust-bindgen */
+
+
+#![allow(non_snake_case)]
+
+
+

libbindgen/tests/headers/type-referenced-by-whitelisted-function.h

@@ -0,0 +1,7 @@
+// bindgen-flags: --whitelist-type dl_iterate_phdr
+
+struct dl_phdr_info {
+    int x;
+};
+
+int dl_iterate_phdr(struct dl_phdr_info *);