st/mesa: automake: honour the vdpau header install location

st/mesa: automake: honour the vdpau header install location

If VDPAU is installed in the non-default location, we'll fail to find
the headers and error at build time.

../../src/gallium/include/state_tracker/vdpau_dmabuf.h:37:25: fatal error: vdpau/vdpau.h: No such file or directory
 #include <vdpau/vdpau.h>
                         ^

Fixes: faba96bc60b ("st/vdpau: add new interop interface")
Cc: Christian König <christian.koenig@amd.com>
Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Nicolai Hähnle <nicolai.haehnle@amd.com>