vulkan: EXT_acquire_xlib_display requires libXrandr headers to build

vulkan: EXT_acquire_xlib_display requires libXrandr headers to build

When VK_USE_PLATFORM_XLIB_XRANDR_EXT is defined, vulkan.h includes
X11/extensions/Xrandr.h for the RROutput typedef which is used in
the vkGetRandROutputDisplayEXT interface.

Make sure we have the required header by checking during the build,
and also set CFLAGS to point at the right directory.

We don't need to link against the library as we don't use any
functions from there, so don't add the _LIBS value in the autotools
build.

Signed-off-by: Keith Packard <keithp@keithp.com>
Fixes: dbac8e25f851ed44c51f "radv: Add EXT_acquire_xlib_display to radv driver [v2]"
Reviewed-by: Eric Engestrom <eric.engestrom@intel.com>