Fix GDB compilation on MinGW (PR gdb/24292)

gdb/ChangeLog:

2019-03-02  Eli Zaretskii  <eliz@gnu.org>

	PR gdb/24292
	* common/netstuff.c:
	* gdbserver/gdbreplay.c
	* gdbserver/remote-utils.c:
	* ser-tcp.c:
	* unittests/parse-connection-spec-selftests.c [USE_WIN32API]:
	Include ws2tcpip.h instead of wsiapi.h and winsock2.h.  Redefine
	_WIN32_WINNT to 0x0501 if defined to a smaller value, as
	'getaddrinfo' and 'freeaddrinfo' were not available before
	Windows XP, and mingw.org's MinGW headers by default define
	_WIN32_WINNT to 0x500.