tests: include foo.h in foo.c

Fixes:

      CC       foo.lo
    /home/smarchi/src/lttng-tools/tests/utils/testapp/userspace-probe-elf-binary/foo.c:19:5: error: no previous declaration for ‘dynamic_symbol’ [-Werror=missing-declarations]
     int dynamic_symbol(int a)
         ^~~~~~~~~~~~~~

Change-Id: I58888da2908bcf4c38d59ba9bacf3e96adc575b6
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>