src: make 'nothing' target a static library

This avoids the creation of `nothing.node` files in other projects.
These files will otherwise be identified as non-N-API modules, even
though they are empty.

PR-URL: #348
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>
Reviewed-By: Nicola Del Gobbo <nicoladelgobbo@NickNaso.local>

Author: Gabriel Schulhof

src/node_api.gyp

@@ -1,7 +1,9 @@
 {
   'targets': [
     {
-      'target_name': 'nothing'
+      'target_name': 'nothing',
+      'type': 'static_library',
+      'sources': [ 'nothing.c' ]
     },
     {
       'target_name': 'node-api',