libsframe: bfd: use uint32_t for return type of get_num_fidx APIs

Keep the data types usage in libsframe look consistent.

bfd/
	* elf-sframe.c (_bfd_elf_merge_section_sframe): Use uint32_t
	type alias.
	* libsframe/sframe.c (sframe_decoder_get_funcdesc_at_index):
	Likewise.
	(sframe_decoder_get_num_fidx): Likewise.
	(sframe_encoder_get_num_fidx): Likewise.
include/
	* sframe-api.h (sframe_decoder_get_num_fidx): Likewise.
	(sframe_encoder_get_num_fidx): Likewise.