Fix missing #include in nacl_entry_points.h
Fix missing #include in nacl_entry_points.h

Attempt to fix the error that occurred on some of the Chromium Linux
buildbots when updating DEPS to include r2744:

./native_client/src/trusted/plugin/nacl_entry_points.h:52: error: ‘string’ is not a member of ‘std’

Review URL: http://codereview.chromium.org/3034022

git-svn-id: 721b910a23eff8a86f00c8fd261a7587cddf18f8@2776 fcba33aa-ac0c-11dd-b9e7-8d5594d729c2

sbc100/native_client,sbc100/native_client,nacl-webkit/native_client,nacl-webkit/native_client,sbc100/native_client,sbc100/native_client,sbc100/native_client,nacl-webkit/native_client,nacl-webkit/native_client,sbc100/native_client,nacl-webkit/native_client