ASoC: intel - atom: fix sst_platform_compr_ops declaration

Sparse complains that sst_platform_compr_ops should be static, but the
declaration of this symbol was not correct so declare the symbol as
extern in header file

sound/soc/intel/atom/sst-mfld-platform-compress.c:257:22: warning: symbol
'sst_platform_compr_ops' was not declared. Should it be static?

Signed-off-by: Vinod Koul <vinod.koul@intel.com>
Acked-by: Liam Girdwood <liam.r.girdwood@linux.intel.com>
Signed-off-by: Mark Brown <broonie@kernel.org>