Add missing include to self_check.c.

This fixes the build for folks not using bcm.c.

Change-Id: I47935d8af7cb5a12ff2918ee2a8774182681d930
Reviewed-on: https://boringssl-review.googlesource.com/c/boringssl/+/45384
Commit-Queue: David Benjamin <davidben@google.com>
Commit-Queue: Adam Langley <agl@google.com>
Reviewed-by: Adam Langley <agl@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/dh.h>
 #include <openssl/digest.h>
 #include <openssl/ec.h>
 #include <openssl/ecdsa.h>