Export BN_num_bits_word.

This function was missed when the OPENSSL_EXPORT tags were first added.

Change-Id: Ia73555b8e7ca87f228a8ff9b281d7c401f1655a7
Reviewed-on: https://boringssl-review.googlesource.com/1553
Reviewed-by: David Benjamin <davidben@chromium.org>
Reviewed-by: Adam Langley <agl@google.com>

include/openssl/bn.h

@@ -792,7 +792,7 @@ struct bn_mont_ctx_st {
   int ri;    /* number of bits in R */
 };
 
-unsigned BN_num_bits_word(BN_ULONG l);
+OPENSSL_EXPORT unsigned BN_num_bits_word(BN_ULONG l);
 
 #define BN_FLG_MALLOCED 0x01
 #define BN_FLG_STATIC_DATA 0x02