Fix out-of-tree build

Change-Id: I43910d569d275737cac59b053c71f84bd1dc599d
diff --git a/fuchsia/BUILD.gn b/fuchsia/BUILD.gn
index 2dfa250..1eb978d 100644
--- a/fuchsia/BUILD.gn
+++ b/fuchsia/BUILD.gn
@@ -85,7 +85,7 @@
 
     deps = [
       "//sdk:zircon_sysroot_export",
-      "//src/graphics/lib/magma/src/libmagma:libmagma_complete",
+      "//src/graphics/lib/magma/src/libmagma:libmagma_complete($shlib_toolchain)",
     ]
   }
 }