build: fix CFLAGS for snmp modules

build: fix CFLAGS for snmp modules

The SNMP modules include <net-snmp/net-snmp-config.h>, which won't be
found in off-searchpath directories without SNMP_CFLAGS.  Unfortunately
in my tests the files were on the search path even without the flags.

(SNMP_LIBS is not needed because only libfrrsnmp calls into net-snmp
functions.)

Signed-off-by: David Lamparter <equinox@opensourcerouting.org>