buffer: remove a wrongly added attribute specifier

It doesn't seem to make much sense to have the mentioned typedef
declaration equipped with NODE_EXTERN. In fact, when compiling with GCC,
an attribute specifier like __attribute__((visibility("default"))) in
such a typedef declaration will cause the following warning message:

  warning: ‘visibility’ attribute ignored [-Wattributes]

The issue goes unnoticed because NODE_EXTERN is defined as nothing for
GCC builds, but for correctness it's better to not specify it here at
all.

PR-URL: #14466
Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: Trevor Norris <trev.norris@gmail.com>

src/node_buffer.h

@@ -13,7 +13,7 @@ namespace Buffer {
 static const unsigned int kMaxLength =
     sizeof(int32_t) == sizeof(intptr_t) ? 0x3fffffff : 0x7fffffff;
 
-NODE_EXTERN typedef void (*FreeCallback)(char* data, void* hint);
+typedef void (*FreeCallback)(char* data, void* hint);
 
 NODE_EXTERN bool HasInstance(v8::Local<v8::Value> val);
 NODE_EXTERN bool HasInstance(v8::Local<v8::Object> val);