gyp: use --export-dynamic on FreeBSD

Should help addons use OpenSSL functions.

Reviewed-By: Fedor Indutny <fedor@indutny.com>

common.gypi

@@ -235,6 +235,11 @@
       }],
       ['OS=="freebsd" and node_use_dtrace=="true"', {
         'libraries': [ '-lelf' ],
+      }],
+      ['OS=="freebsd"', {
+        'ldflags': [
+          '-Wl,--export-dynamic',
+        ],
       }]
     ],
   }