Include the standard headers that newer code uses directly

These files use std::function, std::map, std::deque, std::unique_ptr or std::string_view and got
the header through others, which MSVC's standard library doesn't always do.

Core/Core.cpp

@@ -17,6 +17,7 @@
 
 #include "ppsspp_config.h"
 
+#include <functional>
 #include <cstdint>
 #include <deque>
 #include <set>

Core/Dialog/PSPOskDialog.cpp

@@ -17,6 +17,7 @@
 
 #include "ppsspp_config.h"
 
+#include <string_view>
 #include <cctype>
 #include <cmath>
 #include <algorithm>

Core/EmuThread.cpp

@@ -1,5 +1,6 @@
 #include "ppsspp_config.h"
 
+#include <functional>
 #include <retro_atomic.h>
 
 #include "Common/System/System.h"