test: fix build with gcc 4.8.5

test: fix build with gcc 4.8.5

Build errors occur on CentOS 7 with GCC 4.8.5 20150623 in the
event_timer_adapter_test autotest; the
-Werror=missing-field-initializers option causes the compiler to emit
messages like "error: missing initializer for field ‘priority’ of
‘struct <anonymous>’" in several places.

Add -Wno-missing-field-initializers to the test's CFLAGS to allow the
current syntax if we are using GCC 5.0 or lower.

Fixes: d1f3385d0076 ("test: add event timer adapter auto-test")

Signed-off-by: Erik Gabriel Carrillo <erik.g.carrillo@intel.com>
Acked-by: Jerin Jacob <jerin.jacob@caviumnetworks.com>