tests: make entry_handler static in ini_config test

Fixes:

      CC       ini_config.o
    /home/smarchi/src/lttng-tools/tests/unit/ini_config/ini_config.c:37:5: error: no previous declaration for ‘entry_handler’ [-Werror=missing-declarations]
     int entry_handler(const struct config_entry *entry,
         ^~~~~~~~~~~~~

Change-Id: I937fda691d5ab81bf7567ab50fd90903911dfc57
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>