libsframe: add new APIs to get SFrame version

libsframe: add new APIs to get SFrame version

While the SFrame preamble is guaranteed to not change between versions,
providing these access APIs from the SFrame decoder and encoder APIs is
for convenience only.  The linker may want to use these APIs as the
format evolves.

include/
	* sframe-api.h (sframe_decoder_get_version): New declaration.
	(sframe_encoder_get_version): Likewise.

libsframe/
	* libsframe/libsframe.ver: Add new APIs.
	* libsframe/sframe.c (sframe_decoder_get_version): New
	definition.
	(sframe_encoder_get_version): Likewise.