rustc: Fix ICE with #[target_feature] on module

This commit fixes an ICE in rustc when `#[target_feature]` was applied to items
other than functions due to the way the feature was validated.

src/test/ui/target-feature-wrong.rs

@@ -29,6 +29,10 @@ unsafe fn foo() {}
 //~^ ERROR: can only be applied to `unsafe` function
 fn bar() {}
 
+#[target_feature(enable = "sse2")]
+//~^ ERROR: should be applied to a function
+mod another {}
+
 fn main() {
     unsafe {
         foo();