Add missing assert.h include.

Change-Id: I4af18ce3de2a01a8a5184096b07354bcbd24caf1
Reviewed-on: https://boringssl-review.googlesource.com/c/boringssl/+/50265
Reviewed-by: Adam Langley <agl@google.com>
Commit-Queue: David Benjamin <davidben@google.com>

crypto/asn1/tasn_enc.c

@@ -56,6 +56,7 @@
 
 #include <openssl/asn1.h>
 
+#include <assert.h>
 #include <limits.h>
 #include <string.h>