[Ada] Keepalive control on Windows

[Pierre-Marie de Rodat]

Windows headers in GCC could miss some available constants. Hardcode
TCP_KEEPCNT, TCP_KEEPIDLE, and TCP_KEEPINTVL constants for such case.

Tested on x86_64-pc-linux-gnu, committed on trunk

gcc/ada/

	* s-oscons-tmplt.c (TCP_KEEPCNT TCP_KEEPIDLE, TCP_KEEPINTVL):
	Hardcode on Windows if undefined.
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