sflow: Update build rules

sflow: Update build rules

1) we rely on Netlink PSAMPLE and USERSOCK make sure we have headers
2) sflow plugin is not available on FreeBSD, due to Netlink features
3) preprocessor flag SFLOW_USE_VAPI controls whether the sflow plugin
will attempt to contact the linux-cp plugin using the binary VAPI. It
has to be a preprocessor flag so it can suppress the "#include"
statements that define that linux-cp api, because those include files
will be missing if linux-cp is excluded from the build.

The "excluded_plugins" list can be used with:
make VPP_EXCLUDED_PLUGINS=linux-cp build build-release

Type: fix
Fixes: e40f8a90bb0c39986c198fca8ad9b0b3c1658401
Change-Id: If44929d285d27db7862910ecb4ec11ddd4591fd2
Signed-off-by: [email protected]
(cherry picked from commit d9b9a721fd1aa0198b5d0be54fe46ad0a3d773b7)