Fix build when HAVE_LIBCAPNG is not defined.

Fix build when HAVE_LIBCAPNG is not defined.

The function daemon_become_new_user_linux was conditionally defined but
then used in code unconditionally.  If HAVE_LIBCAPNG is not defined, the
function would never be called, but it still must exist.

Adjust the #if guard around the function to be around the body of the
function instead of outside of its definition to ensure the function is
always defined, even if empty.

This issue was introduced in e91b927d8966bfcb9768225392324dde4fd7d7f6.

Signed-off-by: Russell Bryant <rbryant@redhat.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>