Bump LT version

Adding the OPUS_GET_IN_DTX() query
diff --git a/configure.ac b/configure.ac
index e2921fa..18ece29 100644
--- a/configure.ac
+++ b/configure.ac
@@ -22,9 +22,9 @@
 
 # For libtool.
 dnl Please update these for releases.
-OPUS_LT_CURRENT=7
+OPUS_LT_CURRENT=8
 OPUS_LT_REVISION=0
-OPUS_LT_AGE=7
+OPUS_LT_AGE=8
 
 AC_SUBST(OPUS_LT_CURRENT)
 AC_SUBST(OPUS_LT_REVISION)