Make FormatValue constructor explicit

Signed-off-by: Pavel Artsishevsky <polter.rnd@gmail.com>

include/slimlog/format.h

@@ -164,7 +164,7 @@ class FormatValue final {
      * @param formatter Reference to the formatter.
      * @param value Value to be formatted.
      */
-    constexpr FormatValue(const CachedFormatter<T, Char>& formatter, T value)
+    constexpr explicit FormatValue(const CachedFormatter<T, Char>& formatter, T value)
         : m_formatter(formatter)
         , m_value(std::move(value))
     {