[vulkan] Fix build error for linux host build.

Change-Id: I07f9dfd36e5e8c1f1aa88c4576f742ea73e27f47
Reviewed-on: https://fuchsia-review.googlesource.com/c/third_party/Vulkan-Loader/+/435536
Reviewed-by: John Rosasco <rosasco@google.com>
diff --git a/loader/vk_loader_platform.h b/loader/vk_loader_platform.h
index 8fbb271..9e312f9 100644
--- a/loader/vk_loader_platform.h
+++ b/loader/vk_loader_platform.h
@@ -52,6 +52,7 @@
 #include <stdlib.h>
 #include <libgen.h>
 #include <stdio.h>
+#include <unistd.h>
 
 // VK Library Filenames, Paths, etc.:
 #define PATH_SEPARATOR ':'