Fix "since" version for getpid feature.

It was stabilized right before the beta branch was cut for 1.26.0.

See rust-lang#49523 (comment)

src/libstd/process.rs

@@ -1426,7 +1426,7 @@ pub fn abort() -> ! {
 /// ```
 ///
 ///
-#[stable(feature = "getpid", since = "1.27.0")]
+#[stable(feature = "getpid", since = "1.26.0")]
 pub fn id() -> u32 {
     ::sys::os::getpid()
 }