PL/Perl: Avoid compiler warning from clang

Use SvREFCNT_inc_simple_void() instead of SvREFCNT_inc() to avoid
warning about unused return value.

src/pl/plperl/plperl_helpers.h

@@ -64,7 +64,7 @@ sv2cstr(SV *sv)
 	else
 		/* increase the reference count so we can just SvREFCNT_dec() it when
 		 * we are done */
-		SvREFCNT_inc(sv);
+		SvREFCNT_inc_simple_void(sv);
 
 	val = SvPVutf8(sv, len);