app/test: fix flags with meson

app/test: fix flags with meson

in app/test/meson.build the default_cflag is never used so the
-D_GNU_SOURCE was never passed as intended.

Fixes the following build error with musl libc:

lib/librte_eal/common/include/rte_lcore.h:26:9: error:
unknown type name 'cpu_set_t'
 typedef cpu_set_t rte_cpuset_t;
         ^~~~~~~~~

The problem is that cpu_set_t is only defined when _GNU_SOURCE is set.

Fixes: 5d7b673d5fd6 ("mk: build with _GNU_SOURCE defined by default")
Cc: stable@dpdk.org
Signed-off-by: Natanael Copa <ncopa@alpinelinux.org>