syscall: fix bug in mksyscall_windows.pl

This change fixes generation of "shadow" variables for bool parameters.
Before the change, it was naming all bool variables with the same name of _p0.
Now it calls them _p0, _p1, ... So the code could compile.

R=golang-dev, r
CC=golang-dev
https://golang.org/cl/4479047

src/pkg/syscall/mksyscall_windows.pl

@@ -156,6 +156,7 @@ while(<>) {
  			$text .= "\tvar _p$n uint32\n";
 			$text .= "\tif $name {\n\t\t_p$n = 1\n\t} else {\n\t\t_p$n = 0\n\t}\n";
 			push @args, "uintptr(_p$n)";
+			$n++;
 		} else {
 			push @args, "uintptr($name)";
 		}