build: Fix CMake Xlib DisplayServer name.

Change-Id: Ia50157cd4f245971208a1baa28c823a62683dcd3
diff --git a/CMakeLists.txt b/CMakeLists.txt
index c1b89ea..603a4c0 100755
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -34,7 +34,7 @@
 
     if (BUILD_WSI_XLIB_SUPPORT)
         add_definitions(-DVK_USE_PLATFORM_XLIB_KHR)
-        set(DisplayServer XLib)
+        set(DisplayServer Xlib)
     endif()
 
     if (BUILD_WSI_WAYLAND_SUPPORT)