tests: put `static` keyword at beginning of variable declaration

tests: put `static` keyword at beginning of variable declaration

Fix:

  CC       select_poll_epoll-select_poll_epoll.o
/home/smarchi/src/lttng-tools/tests/regression/kernel/select_poll_epoll.c:34:1: error: ‘static’ is not at beginning of declaration [-Werror=old-style-declaration]
 volatile static int stop_thread;
 ^~~~~~~~

Change-Id: I2cf1a7d50a01ca0e9226ea527f2e8af70a292f94
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>