smalloc: remove `env.h` include

Since `smalloc.h` is included in a `node_buffer.h`, including private
headers in it is strictly prohibited.

fix #7206

src/smalloc.h

@@ -24,10 +24,12 @@
 
 #include "node.h"
 #include "v8.h"
-#include "env.h"
 
 namespace node {
 
+// Forward declaration
+class Environment;
+
 /**
  * Simple memory allocator.
  *