opusdec: avoid __STDC_VERSION__ warning.

opusdec: avoid __STDC_VERSION__ warning.

opusdec_filter.c indirectly includes opus_types.h which checks whether
__STDC_VERSION__ >= 199901L. However, at least some gcc versions
don't define __STDC_VERSION__ which results in

	warning: "__STDC_VERSION__" is not defined

For example, this happens on Ubuntu lucid, which ships gcc-4.4.3.

This patch gets rid of the warning by defining __STDC_VERSION__
if necessary prior to including the opus_types header.