[Ada] Fix classification of Subprogram_Variant as assertion pragma

[Pierre-Marie de Rodat via Gcc-patches]

This pragma was wrongly not recognized as an assertion pragma.  Now
fixed.

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

gcc/ada/

        * sem_prag.ads (Assertion_Expression_Pragmas): Fix value for
        pragma Subprogram_Variant.
diff --git a/gcc/ada/sem_prag.ads b/gcc/ada/sem_prag.ads
--- a/gcc/ada/sem_prag.ads
+++ b/gcc/ada/sem_prag.ads
@@ -149,6 +149,7 @@ package Sem_Prag is
       Pragma_Precondition              => True,
       Pragma_Predicate                 => True,
       Pragma_Refined_Post              => True,
+      Pragma_Subprogram_Variant        => True,
       Pragma_Test_Case                 => True,
       Pragma_Type_Invariant            => True,
       Pragma_Type_Invariant_Class      => True,