rpc: fix compilation with boost 1.87 #3191

Job	Run time
Lint	50s
macOS 13 native [GOAL install] [GUI] [no depends]	10m 6s
ARM [GOAL install] [bullseye]	2m 37s
Win32	3m 14s
Win64	4m 32s
i386 Linux [GOAL install] [focal]	2m 12s
x86_64 Linux [GOAL install] [GUI] [focal] [no depends]	14m 52s
x86_64 Linux [GOAL install] [GUI] [noble] [no depends]	13m 34s
x86_64 Linux [ASan] [LSan] [UBSan] [integer] [jammy] [no depends]	5m 20s
macOS 10.14 [GOAL deploy] [GUI] [no tests] [noble]	3m 30s
arm64 macOS 10.14 [GOAL deploy] [GUI] [no tests] [noble]	3m 29s
	1h 4m 16s

Provide feedback