[Ada] Fix Ada-QNX task priority conversion

[Pierre-Marie de Rodat via Gcc-patches]

The conversion between OS and Ada priorties should be done in the wider
Interfaces.C.int type rather than Any_Priority otherwise
Constraint_Error will be raised when coverting Any_Priority'Last to int.

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

gcc/ada/

        * libgnarl/s-osinte__qnx.adb (To_Target_Priority): Perform
        arithmetic in int.
diff --git a/gcc/ada/libgnarl/s-osinte__qnx.adb b/gcc/ada/libgnarl/s-osinte__qnx.adb
--- a/gcc/ada/libgnarl/s-osinte__qnx.adb
+++ b/gcc/ada/libgnarl/s-osinte__qnx.adb
@@ -87,7 +87,7 @@ package body System.OS_Interface is
      (Prio : System.Any_Priority) return Interfaces.C.int
    is
    begin
-      return Interfaces.C.int (Prio + 1);
+      return Interfaces.C.int (Prio) + 1;
    end To_Target_Priority;
 
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