anv: automake: use the local headers over any system provided ones

anv: automake: use the local headers over any system provided ones

At the moment, we would honour any system headers - vulkan_intel.h in
particular over the ones in-tree.

Thus, if one does incremental build of mesa, without the vulkan.h
already installed (or at least not in the same directory as
vulkan_intel.h) the build will fail.

In the future we might want to upstream the vulkan_intel.h within
vulkan.h or use other ways to make vulkan_intel.h obsolete. In either
case, the more robust thing is to rely on our own copy.

v2: Move AM_CPPFLAGS just above LIBDRM_CFLAGS (Grazvydas, Jason)

Tested-by: Grazvydas Ignotas <notasas@gmail.com>
Fixes: ee8044fd "intel/vulkan: Get rid of recursive make"
Suggested-by: Jason Ekstrand <jason@jlekstrand.net>
Reported-by: Grazvydas Ignotas <notasas@gmail.com>
Signed-off-by: Emil Velikov <emil.velikov@collabora.com>