Add digest.h to self_check.c

This covers the use of EVP_sha256() added in 8846533744.

Change-Id: I8cd4c8e271de6a0b9a926e7186c7b24ffe849d67
Reviewed-on: https://boringssl-review.googlesource.com/c/boringssl/+/44224
Commit-Queue: Adam Langley <agl@google.com>
Commit-Queue: David Benjamin <davidben@google.com>
Reviewed-by: David Benjamin <davidben@google.com>

crypto/fipsmodule/self_check/self_check.c

@@ -21,6 +21,7 @@
 #include <openssl/aes.h>
 #include <openssl/bn.h>
 #include <openssl/des.h>
+#include <openssl/digest.h>
 #include <openssl/ec.h>
 #include <openssl/ecdsa.h>
 #include <openssl/ec_key.h>