vulkan: Allow drivers to add private gralloc usage flags

Change-Id: Iea33990b3a85a761e27d54bb323d866267c88901
(cherry picked from commit c040fbedde52d632a7ceeef3a9266d35b22ce53f)
diff --git a/vulkan/libvulkan/loader.h b/vulkan/libvulkan/loader.h
index ef3d1f1..74694a8 100644
--- a/vulkan/libvulkan/loader.h
+++ b/vulkan/libvulkan/loader.h
@@ -202,6 +202,7 @@
     PFN_vkQueuePresentKHR QueuePresentKHR;
 
     // Implemented only by drivers, not by layers or the loader
+    PFN_vkGetSwapchainGrallocUsageANDROID GetSwapchainGrallocUsageANDROID;
     PFN_vkImportNativeFenceANDROID ImportNativeFenceANDROID;
     PFN_vkQueueSignalNativeFenceANDROID QueueSignalNativeFenceANDROID;
 };