meson: use absolute paths for doxygen stamp files

The relative paths are incorrect when running as a subproject.

Signed-off-by: Simon Ser <contact@emersion.fr>
3 jobs for meson-doxygen-subproject in 3 minutes and 3 seconds (queued for 6 seconds)
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Status Job ID Name Coverage
  Prep
passed #6463517
debian:buster@container-prep

00:01:46

 
  Build
passed #6463518
build-native-autotools

00:01:16

passed #6463519
build-native-meson

00:00:30