[COFF] Always include the size of the string table size field

Even if we don't actually write any string table contents, the
4 byte size for the string table will always be written. Make
sure we accommodate for this in the file size. Since this size
is aligned up, this would seldom be an issue in practice.

Differential Revision: https://reviews.llvm.org/D39891

llvm-svn: 318284