test: fix _embdemo write test

_embdemo/write-esp32/main.go

@@ -1,6 +1,12 @@
 package main
 
-import "github.com/goplus/lib/c"
+import (
+	_ "unsafe"
+	"github.com/goplus/lib/c"
+)
+
+//go:linkname write C.write
+func write(fd c.Int, *c.Char, c.SizeT) int
 
 func main() {
 	buf := c.Malloc(6)
@@ -8,5 +14,4 @@ func main() {
 	c.Strncpy((*c.Char)(buf), c.Str("abcde"), 5)
 
 	write(1, (*c.Char)(buf), 5)
-
 }