bgpd: fix automake fart under MSAN

bgpd: fix automake fart under MSAN

"When building without VNC, automake sees that the `bgpd_bgpd_CFLAGS`
variable exists, although it is only set in the VNC-enabled case... but
since the variable exists, it unconditionally drops `AM_CFLAGS` for the
two bgp targets and uses `bgpd_bgpd_CFLAGS` instead, which will
contain... _nothing_."

This was breaking builds of bgpd binaries with MSAN enabled.

Signed-off-by: David Lamparter <equinox@diac24.net>
Signed-off-by: Quentin Young <qlyoung@cumulusnetworks.com>