GN: Add missing header dependencies.

These headers were detected as missing by an ANGLE tool. Adding them
should fix some edge cases with incremental builds.
diff --git a/BUILD.gn b/BUILD.gn
index ef25a39..121b436 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -74,6 +74,7 @@
 
   target(library_type, "libvulkan") {
     sources = [
+      "loader/adapters.h",
       "loader/asm_offset.c",
       "loader/cJSON.c",
       "loader/cJSON.h",
@@ -82,6 +83,9 @@
       "loader/dev_ext_trampoline.c",
       "loader/extension_manual.c",
       "loader/extension_manual.h",
+      "loader/generated/vk_layer_dispatch_table.h",
+      "loader/generated/vk_loader_extensions.h",
+      "loader/generated/vk_object_types.h",
       "loader/gpa_helper.h",
       "loader/loader.c",
       "loader/loader.h",
@@ -89,6 +93,7 @@
       "loader/murmurhash.h",
       "loader/phys_dev_ext.c",
       "loader/trampoline.c",
+      "loader/vk_loader_layer.h",
 
       # TODO(jmadill): Use assembler where available.
       "loader/unknown_ext_chain.c",