meson.build: use local include path for tools

Fixes the following compile failure with old toolchains and meson
build (autotools build not affected):

  ../tools/mouse-dpi-tool.c: In function ‘handle_event’:
  ../tools/mouse-dpi-tool.c:115:13: error: ‘const struct input_event’ has no member named ‘input_event_sec’
     m->us = ev->input_event_sec * 1000000 + ev->input_event_usec;
               ^
  ../tools/mouse-dpi-tool.c:115:45: error: ‘const struct input_event’ has no member named ‘input_event_usec’
     m->us = ev->input_event_sec * 1000000 + ev->input_event_usec;
                                               ^

Signed-off-by: Peter Seiderer <ps.report@gmx.net>
Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>