Add missing CALLBACK modifier.

diff --git a/tools/winebuild/spec16.c b/tools/winebuild/spec16.c
index e345dcc..214941e 100644
--- a/tools/winebuild/spec16.c
+++ b/tools/winebuild/spec16.c
@@ -434,7 +434,7 @@
         exit(1);
     }
 
-    fprintf( outfile, "%s %s_CallTo16_%s( FARPROC16 proc",
+    fprintf( outfile, "%s CALLBACK %s_CallTo16_%s( FARPROC16 proc",
              short_ret? "WORD" : "LONG", prefix, profile );
     args = profile + 5;
     for ( i = 0; args[i]; i++ )