Fixed missing #include <pjmedia/errno.h> in nullsound.c

git-svn-id: https://svn.pjsip.org/repos/pjproject/trunk@772 74dad513-b988-da41-8d7b-12977e46ad98
diff --git a/pjmedia/src/pjmedia/nullsound.c b/pjmedia/src/pjmedia/nullsound.c
index dd6a1cb..fccc863 100644
--- a/pjmedia/src/pjmedia/nullsound.c
+++ b/pjmedia/src/pjmedia/nullsound.c
@@ -17,6 +17,7 @@
  * Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA 
  */
 #include <pjmedia/sound.h>
+#include <pjmedia/errno.h>
 #include <pj/assert.h>
 
 #if PJMEDIA_SOUND_IMPLEMENTATION==PJMEDIA_SOUND_NULL_SOUND