build: fix dependencies on AIX

Addon tests were starting to run before the node exp file
creation was complete. Add process_outputs_as_sources to avoid
this.

Fixes: https://github.com/nodejs/node/issues/8239
PR-URL: https://github.com/nodejs/node/pull/8272
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Johan Bergström <bugs@bergstroem.nu>

node.gyp

@@ -731,6 +731,7 @@
         },
         {
           'target_name': 'node_exp',
+          'process_outputs_as_sources': 1,
           'type': 'none',
           'dependencies': [
             '<(node_core_target_name)',