rpc: fix compilation with boost 1.87 #3190

Job	Run time
Lint	47s
ARM [GOAL install] [bullseye]	12m 48s
Win32	14m 10s
Win64	31m 49s
macOS 13 native [GOAL install] [GUI] [no depends]	17m 21s
i386 Linux [GOAL install] [focal]	13m 44s
x86_64 Linux [GOAL install] [GUI] [focal] [no depends]	16m 31s
x86_64 Linux [GOAL install] [GUI] [noble] [no depends]	14m 5s
x86_64 Linux [ASan] [LSan] [UBSan] [integer] [jammy] [no depends]	19m 26s
macOS 10.14 [GOAL deploy] [GUI] [no tests] [noble]	28m 17s
arm64 macOS 10.14 [GOAL deploy] [GUI] [no tests] [noble]	27m 14s
	3h 16m 12s

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