Support BigNum::to_vec_padded() with boringssl

The BN_bn2binpad() function is available in boringssl so enable
BigNum::to_vec_padded() which makes use of it.

Test: use the function
Change-Id: I1df5875ab9afc2efd2edbd8305456355229bda71
diff --git a/openssl/src/bn.rs b/openssl/src/bn.rs
index c15ab53..5740557 100644
--- a/openssl/src/bn.rs
+++ b/openssl/src/bn.rs
@@ -809,7 +809,7 @@
     /// assert_eq!(&bn_vec, &[0, 0, 0x45, 0x43]);
     /// ```
     #[corresponds(BN_bn2binpad)]
-    #[cfg(ossl110)]
+    #[cfg(any(boringssl, ossl110))]
     pub fn to_vec_padded(&self, pad_to: i32) -> Result<Vec<u8>, ErrorStack> {
         let mut v = Vec::with_capacity(pad_to as usize);
         unsafe {