Add DSA_bits() function.

RT#4637

Reviewed-by: Rich Salz <rsalz@openssl.org>
diff --git a/crypto/dsa/dsa_lib.c b/crypto/dsa/dsa_lib.c
index 9c001d7..42324c7 100644
--- a/crypto/dsa/dsa_lib.c
+++ b/crypto/dsa/dsa_lib.c
@@ -339,3 +339,8 @@
 {
     return d->engine;
 }
+
+int DSA_bits(const DSA *dsa)
+{
+    return BN_num_bits(dsa->p);
+}