makedef.pl: Properly define a hash element

I had been so used to working lately with C preprocessor directives that
I used the same paradigm in Perl  without noticing.  If 'warnings' had
been on, it would have caught this (the previous commit now turns them
on).

makedef.pl

@@ -195,7 +195,7 @@ BEGIN
                               || (   $define{WIN32}
                                   && $define{USE_THREAD_SAFE_LOCALE})))
 {
-    $define{USE_PERL_SWITCH_LOCALE_CONTEXT}
+    $define{USE_PERL_SWITCH_LOCALE_CONTEXT} = 1;
 }
 
 # perl.h logic duplication ends