Add explicit markup of fallthrough between switch/case labels.

Silences some compiler warnings.

Closes https://github.com/boostorg/log/issues/115.

src/code_conversion.cpp

@@ -125,6 +125,7 @@ inline std::size_t code_convert(const SourceCharT* begin, const SourceCharT* end
                 // Looks like the tail of the source buffer contains only part of the last character.
                 // In this case we intentionally fall through to throw an exception.
             }
+            BOOST_FALLTHROUGH;
 
         default: // std::codecvt_base::error
             BOOST_LOG_THROW_DESCR(conversion_error, "Could not convert character encoding");