LINUX: Fix compile error.

Removal of utils/StdString.h in 6c3a7b62 lost an indirect include of
stdio.h which defined FILE for this header.

I observed this from a clean build on Ubuntu 14.04.  Presumably jenkins hasn't been failing for the last month and a half, though.  So I'm not sure what build env differences brought this on.  The fix is innocuous enough.