Fix Cygwin build after bcb9251f

Absent _UNICODE being defined (which gdb's Makefile doesn't do),
windows.h will always define STARTUPINFO is as STARTUPINFOA, so this
cast isn't correct when create_process expects a STARTUPINFOW
parameter (i.e. in a Cygwin build).

Instead write this as &info_ex.StartupInfo (which is always of the
correct type).