[gn build] Fix defines define on Windows

On Windows, we won't go into the `host_os != "win"` block, so `defines`
won't have been defined, and we'll run into an undefined identifier
error when we try to later append to it. Unconditionally define it at
the start and append to it everywhere else.

Differential Revision: https://reviews.llvm.org/D55617

llvm-svn: 348993