tools: Fixes for intrinsic-gen on windows

Fix slash-related issues.

Change-Id: I618649578746450df7cb2dcd372c5a778eb719ba
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/56776
Kokoro: Kokoro <noreply+kokoro@google.com>
Commit-Queue: Ben Clayton <bclayton@google.com>
Reviewed-by: Antonio Maiorano <amaiorano@google.com>
diff --git a/tools/src/glob/glob.go b/tools/src/glob/glob.go
index e4c8327..b9790b2 100644
--- a/tools/src/glob/glob.go
+++ b/tools/src/glob/glob.go
@@ -192,6 +192,7 @@
 	if err != nil {
 		return false
 	}
+	relPath = filepath.ToSlash(relPath) // Canonicalize
 
 	res := false
 	for _, rule := range c.Paths {