Mark operator c2_cntr64_t as constexpr
am: a52cd7fb5a

Change-Id: If2d333257d24446d0ba3ca2b3297b73fd7b87343
diff --git a/codec2/include/C2.h b/codec2/include/C2.h
index 8a55f8d..ef3466d 100644
--- a/codec2/include/C2.h
+++ b/codec2/include/C2.h
@@ -289,7 +289,7 @@
      * Convert to a smaller counter type. This is always safe.
      */
     template<typename U, typename E=typename std::enable_if<(sizeof(U) < sizeof(T))>::type>
-    inline operator c2_cntr_t<U>() {
+    inline constexpr operator c2_cntr_t<U>() {
         return c2_cntr_t<U>(mValue);
     }