[Silabs] Add build flag for subscription handler

[mkardous-silabs]

Description

Flag for the subscription handler was removed in the refactor. Re-adding it

Test

Manual test to validate it still works

[github-actions]

PR #25020: Size comparison from b04c8ca to 6dd9235

Increases (1 build for cc32xx)

platform	target	config	section	b04c8ca	6dd9235	change	% change
cc32xx	lock	CC3235SF_LAUNCHXL	.debug_info	20188403	20188405	2	0.0

Full report (1 build for cc32xx)

platform	target	config	section	b04c8ca	6dd9235	change	% change
cc32xx	lock	CC3235SF_LAUNCHXL		0	0	0	0.0
			(read only)	640361	640361	0	0.0
			(read/write)	204084	204084	0	0.0
			.ARM.attributes	44	44	0	0.0
			.ARM.exidx	8	8	0	0.0
			.bss	197488	197488	0	0.0
			.comment	194	194	0	0.0
			.data	1476	1476	0	0.0
			.debug_abbrev	928461	928461	0	0.0
			.debug_aranges	87352	87352	0	0.0
			.debug_frame	299840	299840	0	0.0
			.debug_info	20188403	20188405	2	0.0
			.debug_line	2649926	2649926	0	0.0
			.debug_loc	2786018	2786018	0	0.0
			.debug_ranges	280728	280728	0	0.0
			.debug_str	3002856	3002856	0	0.0
			.ramVecs	780	780	0	0.0
			.resetVecs	64	64	0	0.0
			.rodata	105633	105633	0	0.0
			.shstrtab	232	232	0	0.0
			.stab	204	204	0	0.0
			.stabstr	441	441	0	0.0
			.stack	2048	2048	0	0.0
			.strtab	375840	375840	0	0.0
			.symtab	255856	255856	0	0.0
			.text	532604	532604	0	0.0