PerlIO-via: fix PROTOTYPES typo

This line:

    PROTOTYPES: ENABLE;

is actually an error: such declarations shouldn't end with a semicolon.
However, the XS parser has historically treated anything it doesn't
recognise following the PROTOTYPES: keyword as if was 'DISABLE'.

So this commit changes it to  'DISABLE'. This should make no functional
difference, but it will avoid this module starting to warn when ParseXS
is updated shortly to be stricter.

Author: iabyn

ext/PerlIO-via/via.pm

@@ -1,5 +1,5 @@
 package PerlIO::via;
-our $VERSION = '0.19';
+our $VERSION = '0.20';
 require XSLoader;
 XSLoader::load();
 1;

ext/PerlIO-via/via.xs

@@ -704,7 +704,7 @@ static PERLIO_FUNCS_DECL(PerlIO_object) = {
 #endif /* Layers available */
 
 MODULE = PerlIO::via	PACKAGE = PerlIO::via
-PROTOTYPES: ENABLE;
+PROTOTYPES: DISABLE
 
 BOOT:
 {