Add Vulkan Headers repo

We must pull in the Vulkan headers separately due to
upstream repo split.

Change-Id: I83cf0c0c0eedf6384c477b4b9c16c53b46f3e185

default.xml

@@ -373,6 +373,7 @@
   <project path="external/vixl" name="platform/external/vixl" groups="pdk" />
   <project path="external/vogar" name="platform/external/vogar" groups="pdk" />
   <project path="external/volley" name="platform/external/volley" groups="pdk" />
+  <project path="external/vulkan-headers" name="platform/external/vulkan-headers" groups="pdk" />
   <project path="external/vulkan-validation-layers" name="platform/external/vulkan-validation-layers" groups="pdk" />
   <project path="external/walt" name="platform/external/walt" groups="pdk" />
   <project path="external/webp" name="platform/external/webp" groups="pdk,qcom_msm8x26" />