cmake: Add xxhash.c to layer_support_files

Needed by VLF in the downstream VulkanTools repo.

Change-Id: I7da1c7945cbd3491f3a52151a214c59a1736d77c

Author: mark-lunarg

layers/CMakeLists.txt

@@ -62,6 +62,7 @@ if(BUILD_LAYER_SUPPORT_FILES)
         vk_layer_utils.cpp
         vk_loader_platform.h
         xxhash.h
+        xxhash.c
         generated/vk_validation_error_messages.h
         generated/vk_layer_dispatch_table.h
         generated/vk_dispatch_table_helper.h