[Ada] Fix Warn_On_Late_Primitives messages not being properly tagged

[Pierre-Marie de Rodat via Gcc-patches]

This caused tools ingesting GNAT's output to mislabel these messages.

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

gcc/ada/

        * sem_disp.adb (Warn_On_Late_Primitive_After_Private_Extension):
        Fix insertion character.
diff --git a/gcc/ada/sem_disp.adb b/gcc/ada/sem_disp.adb
--- a/gcc/ada/sem_disp.adb
+++ b/gcc/ada/sem_disp.adb
@@ -1207,7 +1207,7 @@ package body Sem_Disp is
                   Error_Msg_Name_2 := Chars (E);
                   Error_Msg_Sloc := Sloc (E);
                   Error_Msg_N
-                    ("?j?primitive of type % defined after private extension "
+                    ("?.j?primitive of type % defined after private extension "
                      & "% #?", Prim);
                   Error_Msg_Name_1 := Chars (Prim);
                   Error_Msg_Name_2 := Chars (E);