x86/net: only select BPF_JIT when NET is enabled

Fix kconfig unmet dependency warning: HAVE_BPF_JIT depends on NET, so
make the "select" of it depend on NET also.

warning: (X86) selects HAVE_BPF_JIT which has unmet direct dependencies (NET)

Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>