[Ada] Add missed OS constant values

[Pierre-Marie de Rodat]

Add IPV6_FLOWINFO and IF_NAMESIZE values into generated package
System.OS_Constants.

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

gcc/ada/

	* gsocket.h: Include net/if.h to get IF_NAMESIZE constant.
	* s-oscons-tmplt.c: Define IPV6_FLOWINFO for Linux.
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