Add ctx.Windows()

Replace ctx.Os() == android.Windows with ctx.Windows()

Test: builds
Change-Id: I7c24c39bcf54a51ad152b21c0021e40a0c07f189
diff --git a/cc/library.go b/cc/library.go
index 953c37a..1871696 100644
--- a/cc/library.go
+++ b/cc/library.go
@@ -228,7 +228,7 @@
 	// MinGW spits out warnings about -fPIC even for -fpie?!) being ignored because
 	// all code is position independent, and then those warnings get promoted to
 	// errors.
-	if ctx.Os() != android.Windows {
+	if !ctx.Windows() {
 		flags.CFlags = append(flags.CFlags, "-fPIC")
 	}