test/ipsec: fix build with GCC 12

test/ipsec: fix build with GCC 12

GCC 12 raises the following warning:

In function ‘_mm256_loadu_si256’,
    inlined from ‘rte_mov32’ at
        ../lib/eal/x86/include/rte_memcpy.h:319:9,
    inlined from ‘rte_mov128’ at
        ../lib/eal/x86/include/rte_memcpy.h:344:2,
    inlined from ‘rte_memcpy_generic’ at
        ../lib/eal/x86/include/rte_memcpy.h:438:4,
    inlined from ‘rte_memcpy’ at
        ../lib/eal/x86/include/rte_memcpy.h:882:10,
    inlined from ‘setup_test_string.constprop’ at
        ../app/test/test_ipsec.c:572:4:
/usr/lib/gcc/x86_64-redhat-linux/12/include/avxintrin.h:929:10: error:
    array subscript ‘__m256i_u[3]’ is partly outside array bounds of
    ‘const char[108]’ [-Werror=array-bounds]
  929 |   return *__P;
      |          ^~~~
../app/test/test_ipsec.c: In function ‘setup_test_string.constprop’:
../app/test/test_ipsec.c:539:12: note: at offset 96 into object
    ‘null_plain_data’ of size 108
  539 | const char null_plain_data[] =
      |            ^~~~~~~~~~~~~~~

Add a hint so that the compiler understands the copied data is within
the passed string boundaries.

Bugzilla ID: 848
Fixes: 05fe65eb66b2 ("test/ipsec: introduce functional test")
Cc: stable@dpdk.org
Signed-off-by: David Marchand <david.marchand@redhat.com>