config: add missing header for libc++ after 3a631e40db78 (#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;
                               ^

1 files changed, 1 insertions, 0 deletions

diff --git a/src/config/ConfigManager.cpp b/src/config/ConfigManager.cpp
index e274f89d..9ef2d1d8 100644
--- a/src/config/ConfigManager.cpp
+++ b/src/config/ConfigManager.cpp
@@ -9,6 +9,7 @@
 #include <algorithm>
 #include <fstream>
 #include <iostream>
+#include <sstream>
 
 extern "C" char** environ;