Fix compilation with mingw.org's MinGW.

Fix compilation with mingw.org's MinGW.

That flavor of MinGW assumes Windows 9X as the default target,
which doesn't expose CONSOLE_FONT_INFO stuff in Windows header
files.  Since we no longer support running on Windows older
than XP anyway, requiring it at build time makes sense.

gdb/ChangeLog
2019-05-03  Eli Zaretskii  <eliz@gnu.org>

	* windows-nat.c [_WIN32_WINNT]: Define _WIN32_WINNT to Windows XP
	level, so that various Windows header files expose the necessary
	declarations and definitions.