config: add missing header for libc++ after 3a631e40db (#2208)

src/config/ConfigManager.cpp:1980:27: error: implicit instantiation of undefined template 'std::basic_stringstream<char>'
        std::stringstream error;
                          ^
/usr/include/c++/v1/iosfwd:134:32: note: template is declared here
    class _LIBCPP_TEMPLATE_VIS basic_stringstream;
                               ^

src/config/ConfigManager.cpp

@@ -9,6 +9,7 @@
 #include <algorithm>
 #include <fstream>
 #include <iostream>
+#include <sstream>
 
 extern "C" char** environ;