[Ada] Fix typo in error message

[Pierre-Marie de Rodat]

An error message mentions "gnamake", where it meant to mention
"gnatmake".

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

2019-09-18  Tom Tromey  <tro...@adacore.com>

gcc/ada/

        * make.adb (Initialize): Fix typo.
--- gcc/ada/make.adb
+++ gcc/ada/make.adb
@@ -3789,7 +3789,7 @@ package body Make is
 
                if Gprbuild = null then
                   Fail_Program
-                    ("project files are no longer supported by gnamake;" &
+                    ("project files are no longer supported by gnatmake;" &
                      " use gprbuild instead");
                end if;