[loader] Add trace-engine as a dep

Vulkan icds depend on libtrace-engine.so, pending MA-488.
Temporarily we must ensure that a vulkan app provides this library.

Test:
vim2:run image-pipe-swapchain-tests

The above test does not otherwise provide the lib.

Change-Id: Iee9cb847aa804be826384f738e433df3173a36ba
diff --git a/loader/BUILD.gn b/loader/BUILD.gn
index 7064501..92a941e 100644
--- a/loader/BUILD.gn
+++ b/loader/BUILD.gn
@@ -86,6 +86,10 @@
     "//zircon/public/lib/fdio",
   ]
 
+  # TODO(MA-488) - while libtrace-engine.so is needed by vulkan icd
+  # the app has to provide it, so we add the dep here.
+  deps += [ "//zircon/public/lib/trace-engine" ]
+
   # This is a C only library, but we have to tell the linker driver that
   # explicitly or it assumes we want to dynamically link against the C++
   # standard library.