[Ada] Fix uses of pragma Unreferenced in MinGW runtime unit

[Pierre-Marie de Rodat via Gcc-patches]

GNAT now emits more warnings about pragma Unreferenced.

Tested on x86_64-pc-linux-gnu, committed on trunk

gcc/ada/

        * libgnarl/s-taprop__mingw.adb (Timed_Sleep): Remove "pragma
        Unreferenced" for Result.
        (Timed_Delay): Likewise.
diff --git a/gcc/ada/libgnarl/s-taprop__mingw.adb b/gcc/ada/libgnarl/s-taprop__mingw.adb
--- a/gcc/ada/libgnarl/s-taprop__mingw.adb
+++ b/gcc/ada/libgnarl/s-taprop__mingw.adb
@@ -559,7 +559,6 @@ package body System.Task_Primitives.Operations is
       Abs_Time   : Duration;
 
       Result : Integer;
-      pragma Unreferenced (Result);
 
       Local_Timedout : Boolean;
 
@@ -615,7 +614,6 @@ package body System.Task_Primitives.Operations is
 
       Timedout : Boolean;
       Result   : Integer;
-      pragma Unreferenced (Timedout, Result);
 
    begin
       Write_Lock (Self_ID);