Fix xcode ci

src/lib/pubkey/ounsworth_kem_combiner/ounsworth.cpp

@@ -35,7 +35,7 @@ std::vector<uint8_t> rlen(std::span<const uint8_t> s) {
    // 2. Let x_1, x_2, ..., x_n be the base-256 encoding of x satisfying:
    //     x = sum 28(n-i)x i, for i = 1 to n
    // 3. Let O_i = uint8(x_i), for i = 1 to n
-   const auto be_sz_encoding = store_be<std::array<uint8_t, size_t(sizeof(size_t))>>(x);
+   const auto be_sz_encoding = store_be(x);
    BOTAN_ASSERT_NOMSG(be_sz_encoding.size() >= n);
    std::vector<uint8_t> o_i(be_sz_encoding.end() - n, be_sz_encoding.end());
    // 4. Let O_{n+1} = uint8(n)