Fix -Wreturn-type warning in xenctrlext_stubs.c (#6859) #15270

Job	Run time
Run OCaml tests (ubuntu-22.04, 5.3.0, true)	7m 30s
Run OCaml tests (ubuntu-22.04, 4.14.2, false)	6m 36s
Run OCaml tests (ubuntu-22.04-arm, 4.14.2, true)	5m 56s
	20m 2s

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