Fix |BN_HEX_FMT2|.

It appears to be only used in p256-x86_64_test.cc, which is obviously
64-bit only and do not affected by this. Internal code search doesn't
find any uses and GitHub just finds several thousand copies of bn.h.

Change-Id: If8185bf6275d90efa172c95cb67c62c86a17e394
Reviewed-on: https://boringssl-review.googlesource.com/c/33464
Commit-Queue: David Benjamin <davidben@google.com>
Reviewed-by: David Benjamin <davidben@google.com>

include/openssl/bn.h

diff --git a/include/openssl/bn.h b/include/openssl/bn.h
index c895cc1..c198f4d 100644
--- a/include/openssl/bn.h
+++ b/include/openssl/bn.h
@@ -160,7 +160,7 @@
 #define BN_DEC_FMT1 "%" PRIu32
 #define BN_DEC_FMT2 "%09" PRIu32
 #define BN_HEX_FMT1 "%" PRIx32
-#define BN_HEX_FMT2 "%08" PRIx64
+#define BN_HEX_FMT2 "%08" PRIx32
 #else
 #error "Must define either OPENSSL_32_BIT or OPENSSL_64_BIT"
 #endif