Use fully-qualified icicle::BinarySerializer in merkle_c_api.cpp

[GarmashAlex]

Describe the changes

Aligns the icicle_merkle_proof_get_serialized_size call with the rest of this file by qualifying BinarySerializer with icicle::. While unqualified usage compiles due to a transitive using namespace icicle;, this change enforces local consistency and avoids reliance on transitive namespace pollution, improving readability and future-proofing against header refactors.

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