libsframe: use const qualifier for sframe_header object

libsframe: use const qualifier for sframe_header object

...where applicable.

For the static functions that do not modify the sframe_header object,
use const.  Ditto for local vars too.

libsframe/
        * sframe.c (sframe_get_hdr_size): Use const qualifier.
        (sframe_header_sanity_check_p): Likewise.
        (flip_sframe): Use const for local var.
        (sframe_decode): Likewise.  While at it, use similar looking var
	name.
        (sframe_decoder_get_hdr_size): Use const for local var.
        (sframe_decoder_get_abi_arch): Likewise.
        (sframe_decoder_get_version): Likewise.
        (sframe_decoder_get_fixed_fp_offset): Likewise.
        (sframe_decoder_get_fixed_ra_offset): Likewise.
        (sframe_get_funcdesc_with_addr_internal): Likewise.
        (sframe_decoder_get_num_fidx): Likewise.
        (sframe_encoder_get_hdr_size): Likewise.
        (sframe_encoder_get_abi_arch): Likewise.
        (sframe_encoder_get_version): Likewise.
        (sframe_encoder_get_num_fidx): Likewise.