Fix access level for extern_proc.p_starttime

Fixes: <rdar://problem/31549450>

stdlib/public/Platform/Darwin.swift.gyb

@@ -74,7 +74,8 @@ public let ${prefix}_TRUE_MIN = ${type}.leastNonzeroMagnitude
 // Macros defined in bsd/sys/proc.h that do not import into Swift.
 extension extern_proc {
     // #define p_starttime p_un.__p_starttime
-    @_transparent var p_starttime: timeval {
+    @_transparent
+    public var p_starttime: timeval {
         get { return self.p_un.__p_starttime }
         set { self.p_un.__p_starttime = newValue }
     }