fix core::os::darwin::objc doctests

Author: jbatez

library/core/src/os/darwin/objc.rs

@@ -68,7 +68,10 @@ pub type SEL = *mut objc_selector;
 /// # Example
 ///
 /// ```no_run
-/// let string_class = class!("NSString");
+/// #![feature(darwin_objc)]
+/// use core::os::darwin::objc;
+///
+/// let string_class = objc::class!("NSString");
 /// ```
 #[allow_internal_unstable(rustc_attrs)]
 pub macro class($classname:expr) {{
@@ -91,8 +94,11 @@ pub macro class($classname:expr) {{
 /// # Examples
 ///
 /// ```no_run
-/// let alloc_sel = selector!("alloc");
-/// let init_sel = selector!("initWithCString:encoding:");
+/// #![feature(darwin_objc)]
+/// use core::os::darwin::objc;
+///
+/// let alloc_sel = objc::selector!("alloc");
+/// let init_sel = objc::selector!("initWithCString:encoding:");
 /// ```
 #[allow_internal_unstable(rustc_attrs)]
 pub macro selector($methname:expr) {{