Update ppport.h to not cause warnings with newest Perl versions.

This is just the minimal necessary change; we might want to adopt
later PPPort output instead.

src/pl/plperl/ppport.h

@@ -221,6 +221,7 @@ __DATA__
 /* Replace: 0 */
 #endif
 
+#ifndef PERL_UNUSED_DECL
 #ifdef HASATTRIBUTE
 #if (defined(__GNUC__) && defined(__cplusplus)) || defined(__INTEL_COMPILER)
 #define PERL_UNUSED_DECL
@@ -230,6 +231,7 @@ __DATA__
 #else
 #define PERL_UNUSED_DECL
 #endif
+#endif
 
 #ifndef dNOOP
 #define NOOP (void)0