Fix path of jsoncpp headers.

The prefix is set via CMake. By putting in it the source files too, we were
incompatible with distributions that put them somewhere else.

src/releases.cpp

@@ -16,7 +16,7 @@
 
 #include <iostream>
 #include <sstream>
-#include <jsoncpp/json/json.h>
+#include <json/json.h>
 #include "gitea2rss.hpp"
 
 using std::cout;

src/tags.cpp

@@ -16,7 +16,7 @@
 
 #include <iostream>
 #include <sstream>
-#include <jsoncpp/json/json.h>
+#include <json/json.h>
 #include "gitea2rss.hpp"
 
 using std::cout;