gitmodules: Add Vulkan headers dependency

.gitmodules

@@ -37,3 +37,6 @@
 [submodule "discord-rpc"]
     path = externals/discord-rpc
     url = https://github.com/discordapp/discord-rpc.git
+[submodule "Vulkan-Headers"]
+    path = externals/Vulkan-Headers
+    url = https://github.com/KhronosGroup/Vulkan-Headers.git

externals/Vulkan-Headers

@@ -0,0 +1 @@
+Subproject commit 7f02d9bb810f371de0fe833c80004c34f7ff8c57