rpc: fix compilation with boost 1.87 #3193

Job	Run time
macOS 13 native [GOAL install] [GUI] [no depends]	14m 22s
Lint	42s
ARM [GOAL install] [bullseye]	2m 51s
Win32	3m 16s
Win64	4m 51s
i386 Linux [GOAL install] [focal]	2m 20s
x86_64 Linux [GOAL install] [GUI] [focal] [no depends]	15m 23s
x86_64 Linux [GOAL install] [GUI] [noble] [no depends]	13m 30s
x86_64 Linux [ASan] [LSan] [UBSan] [integer] [jammy] [no depends]	5m 35s
macOS 10.14 [GOAL deploy] [GUI] [no tests] [noble]	4m 5s
arm64 macOS 10.14 [GOAL deploy] [GUI] [no tests] [noble]	3m 42s
	1h 10m 37s

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