src: use visibility("default") exports on POSIX

Electron uses this because Chromium builds with symbols
hidden by default.

Refs:https://github.com/electron/node/commit/
88b4941
Refs: https://github.com/electron/node/commit/
1293d1d7d0c33d3925da11ceccdce4eb2e927a43

PR-URL: #25893
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Gus Caplan <me@gus.host>

Author: nornagon

src/js_native_api.h

@@ -22,7 +22,7 @@
   #ifdef _WIN32
     #define NAPI_EXTERN __declspec(dllexport)
   #else
-    #define NAPI_EXTERN /* nothing */
+    #define NAPI_EXTERN __attribute__((visibility("default")))
   #endif
 #endif
 

src/node.h

@@ -29,7 +29,7 @@
 #   define NODE_EXTERN __declspec(dllimport)
 # endif
 #else
-# define NODE_EXTERN /* nothing */
+# define NODE_EXTERN __attribute__((visibility("default")))
 #endif
 
 #ifdef BUILDING_NODE_EXTENSION