Use size_t cast for pointer subtractions

Signed-off-by: Dave Rodgman <dave.rodgman@arm.com>
diff --git a/library/aesce.c b/library/aesce.c
index f2bdce2..1a0db16 100644
--- a/library/aesce.c
+++ b/library/aesce.c
@@ -300,7 +300,7 @@
          rki + key_len_in_words < rko_end;
          rki += key_len_in_words) {
 
-        size_t iteration = (rki - (uint32_t *) rk) / key_len_in_words;
+        size_t iteration = (size_t) (rki - (uint32_t *) rk) / key_len_in_words;
         uint32_t *rko;
         rko = rki + key_len_in_words;
         rko[0] = aes_rot_word(aes_sub_word(rki[key_len_in_words - 1]));