Fix vulkan dynamic library name for MacOS (#5974)

The vulkan dynamic library name is libvulkan.dylib in macos platform.
diff --git a/iree/hal/vulkan/dynamic_symbols.cc b/iree/hal/vulkan/dynamic_symbols.cc
index 8a4fe46..f8b22e2 100644
--- a/iree/hal/vulkan/dynamic_symbols.cc
+++ b/iree/hal/vulkan/dynamic_symbols.cc
@@ -69,6 +69,8 @@
 static const char* kVulkanLoaderSearchNames[] = {
 #if defined(IREE_PLATFORM_ANDROID)
     "libvulkan.so",
+#elif defined(IREE_PLATFORM_IOS) || defined(IREE_PLATFORM_MACOS)
+    "libvulkan.dylib",
 #elif defined(IREE_PLATFORM_WINDOWS)
     "vulkan-1.dll",
 #else