Change the canonical way to include the C macros.h to <mojo/macros.h>.

mojom/generators/c/templates/header.tmpl.go

Index: mojom/generators/c/templates/header.tmpl.go
diff --git a/mojom/generators/c/templates/header.tmpl.go b/mojom/generators/c/templates/header.tmpl.go
index d0e9b4c97b3debabfb01a33e788c5f90231cd2b6..9dad8376048620686de8ac2f2daa050cd3bdec5a 100644
--- a/mojom/generators/c/templates/header.tmpl.go
+++ b/mojom/generators/c/templates/header.tmpl.go
@@ -15,6 +15,7 @@ const GenerateHeaderFile = `
 
 #include <assert.h>
 #include <math.h>
+#include <mojo/macros.h>
 #include <stdbool.h>
 #include <stdint.h>
 
@@ -26,7 +27,6 @@ const GenerateHeaderFile = `
 #include "mojo/public/c/bindings/string.h"
 #include "mojo/public/c/bindings/validation.h"
 #include "mojo/public/c/system/handle.h"
-#include "mojo/public/c/system/macros.h"
 
 // Imports.
 {{range $import := .Imports -}}