meson.build: use local include path for tests

meson.build: use local include path for tests

Fixes:

  ../test/test-common.h: In function ‘print_event’:
  ../test/test-common.h:97:6: error: ‘const struct input_event’ has no member named ‘input_event_sec’
      ev->input_event_sec,
        ^
  ../test/test-common.h:98:6: error: ‘const struct input_event’ has no member named ‘input_event_usec’
      ev->input_event_usec,
        ^
  ../test/test-common.h:102:6: error: ‘const struct input_event’ has no member named ‘input_event_sec’
      ev->input_event_sec,
        ^
  ../test/test-common.h:103:6: error: ‘const struct input_event’ has no member named ‘input_event_usec’
      ev->input_event_usec,
        ^

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