[patch] fix compilation for undefined PACKET_MEMBERSHIP

[Bernhard Fischer]

Hi,

http://buildroot.uclibc.org/cgi-bin/viewcvs.cgi/*checkout*/trunk/buildroot/package/strace/strace-undef-PACKET_MEMBERSHIP.patch

	* net.c (PACKET_ADD_MEMBERSHIP, PACKET_DROP_MEMBERSHIP): Make
	them conditional.

Fixes compilation error if they are unavailable.

Please apply.