layers: Fix incorrect KHR/KHX ifdefs.

In two places we were checking for the presence of the WIN32 KHR
define instead of the KHX define where the enums and methods are
defined.

Fixes #1593.

Change-Id: I0bf0c2e2ed734ed2c40a44c4cd7d468ba70ca9a4
diff --git a/layers/parameter_validation.cpp b/layers/parameter_validation.cpp
index 0db8be0..8794144 100644
--- a/layers/parameter_validation.cpp
+++ b/layers/parameter_validation.cpp
@@ -1649,7 +1649,7 @@
             device_data->enables.khx_external_memory_fd = true;
         } else if (strcmp(pCreateInfo->ppEnabledExtensionNames[i], VK_KHX_EXTERNAL_SEMAPHORE_FD_EXTENSION_NAME) == 0) {
             device_data->enables.khx_external_semaphore_fd = true;
-#ifdef VK_USE_PLATFORM_WIN32_KHR
+#ifdef VK_USE_PLATFORM_WIN32_KHX
         } else if (strcmp(pCreateInfo->ppEnabledExtensionNames[i], VK_KHX_EXTERNAL_MEMORY_WIN32_EXTENSION_NAME) == 0) {
             device_data->enables.khx_external_memory_win32 = true;
         } else if (strcmp(pCreateInfo->ppEnabledExtensionNames[i], VK_KHX_EXTERNAL_SEMAPHORE_WIN32_EXTENSION_NAME) == 0) {