format: fix logging of a boolean property

The journal callback keeps silence if the property is boolean.
It happens due the mistype in the format definition.

This commit fixes the mistype in the format definition.

Signed-off-by: Alexander Filippov <a.filippov@yadro.com>
Change-Id: Ibc0fc6d0c275d4f8d6f592794813256d529e0ccd
diff --git a/src/format.hpp b/src/format.hpp
index fe8565b..cbece27 100644
--- a/src/format.hpp
+++ b/src/format.hpp
@@ -21,7 +21,7 @@
 template <>
 struct GetFormatType<bool>
 {
-    static constexpr auto format = "%d";
+    static constexpr auto format = "=%d";
 };
 template <>
 struct GetFormatType<char>