net-snmp: add ac_cv_header_valgrind_* to CACHED_CONFIGUREVARS

There have been occasions that net-snmp sees valgrind and then later it's
not available, adding this setting ensures determinism by disabling it by
default

Signed-off-by: Jackie Huang <jackie.huang@windriver.com>
Signed-off-by: Joe MacDonald <joe_macdonald@mentor.com>