anv/android: Fix Autotools build for VK_ANDROID_native_buffer

Changes to vk.xml and anv_entrypoints_gen.py broke the Autotools build
on Android. The changes undef'd the VK_ANDROID_native_buffer entrypoints
in anv_entrypoints.h.

Fix it with CPPFLAGS += -DVK_USE_PLATFORM_ANDROID_KHR.

CC: <mesa-stable@lists.freedesktop.org>
See-Also: 63525ba7 "android: enable VK_ANDROID_native_buffer"
Reviewed-by: Tapani Pälli <tapani.palli@intel.com>
(cherry picked from commit 8e403bc9)