meson: use message() rather than warning()

warning() was only added to the meson interpreter in 0.44 which is
currently the last version. Let's use message() as we are currently
requiring meson > 0.40. Otherwise we get the following error:

	Meson encountered an error in file overlay/meson.build, line 62, column 1:
	Unknown function "warning".

Fixes: 865a47ca ("overlay: parse tracepoints from sysfs to figure out fields' location")
Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Cc: Rhys Kidd <rhyskidd@gmail.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Petri Latvala <petri.latvala@intel.com>
Reviewed-by: Rhys Kidd <rhyskidd@gmail.com>
diff --git a/overlay/meson.build b/overlay/meson.build
index 8b5c52b..546c837 100644
--- a/overlay/meson.build
+++ b/overlay/meson.build
@@ -59,7 +59,7 @@
 		command: [leg, '-P', '-o', '@OUTPUT@', '@INPUT@'])
 	gpu_overlay_src += leg_file
 else
-	warning('leg command not found, disabling overlay; try : apt-get install peg')
+	message('WARNING: leg command not found, disabling overlay; try : apt-get install peg')
 endif
 
 if leg.found() and xrandr.found() and cairo.found()