libexif: add ptest support

adds ptest support for libexif

Ptest was executed on a core-image-minimal:
root@qemux86-64:~# ptest-runner libexif
START: ptest-runner
2023-07-12T15:45
BEGIN: /usr/lib/libexif/ptest
[...]
i====================
All 13 tests passed
(1 test was not run)
====================
[...]
DURATION: 35
END: /usr/lib/libexif/ptest
2023-07-12T15:45
STOP: ptest-runner
TOTAL: 1 FAIL: 0

Signed-off-by: Julien Stephan <jstephan@baylibre.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>