Make validate_url_option_combination static in create.c

Make validate_url_option_combination static in create.c

Fixes:

      CC       commands/lttng-create.o
    /home/smarchi/src/lttng-tools/src/bin/lttng/commands/create.c:640:5: error: no previous declaration for ‘validate_url_option_combination’ [-Werror=missing-declarations]
     int validate_url_option_combination(void)
         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Change-Id: I091b875fb93d4214c47a1c229e6360ec02a0a2f8
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>