test: fix build of external memory test

test: fix build of external memory test

There was a compilation error in test_external_mem.c:

  CC test_external_mem.o
test_external_mem.c: In function ‘test_external_mem’:
test_external_mem.c:375:2: error: ‘for’ loop initial declarations are
                           only allowed in C99 mode

  for (int i = 0; i < n_pages; i++) {
  ^
test_external_mem.c:375:2: note: use option -std=c99 or -std=gnu99 to
                           compile your code

Fixes: b270daa43b3d ("test: support external memory")

Signed-off-by: Dan Gora <dg@adax.com>
Acked-by: Anatoly Burakov <anatoly.burakov@intel.com>