Add missing #include of assert.h

Change-Id: I641284e657ec184f4209392e7c6f86c20400e7b1
Reviewed-on: https://boringssl-review.googlesource.com/17124
Reviewed-by: David Benjamin <davidben@google.com>
Commit-Queue: Matt Braithwaite <mab@google.com>
CQ-Verified: CQ bot account: commit-bot@chromium.org <commit-bot@chromium.org>

crypto/fipsmodule/cipher/aead.c

@@ -14,6 +14,7 @@
 
 #include <openssl/aead.h>
 
+#include <assert.h>
 #include <string.h>
 
 #include <openssl/cipher.h>