build: add ENABLE_WALLET guards to rpc/rpcassets.cpp

[JSanchezFDZ]

Summary

Restore the ability to build with ./configure --disable-wallet on current develop.

PR #436 (Bug/createrawtransaction for asset) added three new RPC handlers in src/rpc/rpcassets.cpp that unconditionally reference wallet-only symbols (CWallet, GetWalletForJSONRPCRequest, COutput, CCoinControl, CInputCoin, CoinType):

• listassetsbalance (lines 815-869)
• listunspentassets (lines 871-1064)
• listassets, mine=true branch (lines 1160-1203)

The RPC command table at the bottom of the file already wraps the registrations for listassetsbalance and listunspentassets in #ifdef ENABLE_WALLET, but the function bodies themselves were left unguarded. Since the file's includes for wallet headers are gated on ENABLE_WALLET (lines 16-20), --disable-wallet builds fail to compile this TU with dozens of "CWallet was not declared in this scope" errors.

Fix

• Wrap the two wallet-only function bodies (listassetsbalance, listunspentassets) in #ifdef ENABLE_WALLET / #endif. The matching table entries are already guarded, so when the bodies are absent the function pointers are simply never referenced.
• In listassets, the function is intentionally mixed: mine=false is wallet-independent (queries passetsdb only), mine=true needs the wallet. Guard only the mine=true branch, and in the #else path throw RPC_METHOD_NOT_FOUND so a client that requests mine=true against a no-wallet node sees a clear error instead of silently getting wrong data.

Test plan

• ./configure --disable-bench --without-gui --disable-wallet, then make rpc/libraptoreum_server_a-rpcassets.o — compiles cleanly (was broken on develop).
• nm on the resulting .o confirms zero unresolved CWallet / GetWalletForJSONRPCRequest symbols.
• ./configure ... --enable-wallet --with-incompatible-bdb, then recompile the same TU — still builds.

Note: a separate, pre-existing link-time failure in src/mapport.cpp (undefined references to readnatpmpresponseorretry / sendnewportmappingrequest from libnatpmp) keeps the no-wallet raptoreumd link from completing on this build host. That's an unrelated NAT-PMP library-ordering issue and not in scope here.

Why this matters

• CI builds with --disable-wallet (e.g. for testing headless/light-node configurations) are currently broken on develop.
• Future Guix-style deterministic builds typically need both --enable-wallet and --disable-wallet paths to work.

Related: #443 (EVM integration, runs into this same restriction during test-suite verification), #444 (the amount_tests overflow fix that lets the test suite report honest pass/fail counts), #445 (broader CFeeRate hardening tracking).