Rename KHX to EXT to match the released specs

GL_KHX_multiview and GL_KHX_device_group don't exist,
they were released as GL_EXT_multiview and GL_EXT_device_group.

Test/spv.deviceGroup.frag

@@ -1,9 +1,9 @@
 #version 450
 
-#extension GL_KHX_device_group : enable
+#extension GL_EXT_device_group : enable
 
 out vec4 color;
 
 void main() {
-    color = vec4(gl_DeviceIndexKHR, 0, 0, 0);
+    color = vec4(gl_DeviceIndex, 0, 0, 0);
 }