Don't include YAML support headers in API headers

They're not necessary to use the API, and pull in Boost and yaml-cpp
dependencies if included.

include/loot/metadata/plugin_metadata.h

@@ -32,7 +32,6 @@
 #include <vector>
 
 #include "loot/api_decorator.h"
-#include "loot/yaml/set.h"
 #include "loot/metadata/file.h"
 #include "loot/metadata/location.h"
 #include "loot/metadata/message.h"

src/gui/state/loot_state.cpp

@@ -24,6 +24,8 @@
 
 #include "gui/state/loot_state.h"
 
+#include <unordered_set>
+
 #include <boost/algorithm/string.hpp>
 #include <boost/filesystem/fstream.hpp>
 #include <boost/format.hpp>