Add mising cmake file from make dist

Thanks to theislander on HA for reporting this.
diff --git a/Makefile.am b/Makefile.am
index 4e3f183..7f905b0 100644
--- a/Makefile.am
+++ b/Makefile.am
@@ -212,6 +212,7 @@
              Makefile.unix \
              CMakeLists.txt \
              config.h.cmake.in \
+             opus_buildtype.cmake \
              opus_config.cmake \
              opus_functions.cmake \
              opus_sources.cmake \