Guarding files_ attribute with #ifndef NDEBUG guard in FilePicker class.

Summary: Adding guards to files_ attribute of FilePicker class. This attribute is used only in DEBUG mode. This fixes build of static_lib in mac.

Test Plan:
make static_lib in mac
make check all in devserver

Reviewers: ljin, igor, sdong

Reviewed By: sdong

Differential Revision: https://reviews.facebook.net/D20163