X11: Fix CMake check for XInput headers

The X11_Xinput_* variables have been removed in recent CMake, leaving
only the X11_Xi_* set.

Related to #1480.

(cherry picked from commit ceb16cb)

CMakeLists.txt

@@ -218,15 +218,15 @@ if (_GLFW_X11)
     endif()
 
     # Check for XInput (modern HID input)
-    if (NOT X11_Xinput_INCLUDE_PATH)
+    if (NOT X11_Xi_INCLUDE_PATH)
         message(FATAL_ERROR "The XInput headers were not found")
     endif()
 
     list(APPEND glfw_INCLUDE_DIRS "${X11_Xrandr_INCLUDE_PATH}"
                                   "${X11_Xinerama_INCLUDE_PATH}"
                                   "${X11_Xkb_INCLUDE_PATH}"
                                   "${X11_Xcursor_INCLUDE_PATH}"
-                                  "${X11_Xinput_INCLUDE_PATH}")
+                                  "${X11_Xi_INCLUDE_PATH}")
 endif()
 
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