Backends: Win32: Change the case of XInput.h include, so it cross-compile nicely on unices. (#4074)

backends/imgui_impl_win32.cpp

@@ -25,7 +25,7 @@
 
 // Using XInput for gamepad (will load DLL dynamically)
 #ifndef IMGUI_IMPL_WIN32_DISABLE_GAMEPAD
-#include <XInput.h>
+#include <xinput.h>
 typedef DWORD (WINAPI *PFN_XInputGetCapabilities)(DWORD, DWORD, XINPUT_CAPABILITIES*);
 typedef DWORD (WINAPI *PFN_XInputGetState)(DWORD, XINPUT_STATE*);
 #endif