tests: put the printf attribute in the header for _test_print_failed()

tests: put the printf attribute in the header for _test_print_failed()

If the printf attribute is only present next to the function's
definition, the compiler only checks the calls in the same compilation
unit.

In order to verify all the calls, annotate the declaration of
_test_print_failed() in gpiod-test.h.

Signed-off-by: Bartosz Golaszewski <bartekgola@gmail.com>