tests/lib/test_trace_ir_ref.c: rename user structure

tests/lib/test_trace_ir_ref.c: rename user structure

Rename user structure to bt_user to avoid the following build failure
with uclibc:

test_trace_ir_ref.c:41:8: error: redefinition of 'struct user'
 struct user {
        ^
In file included from /home/naourr/work/instance-0/output-1/per-package/babeltrace2/host/arm-buildroot-linux-uclibcgnueabi/sysroot/usr/include/sys/procfs.h:33,
                 from /home/naourr/work/instance-0/output-1/per-package/babeltrace2/host/arm-buildroot-linux-uclibcgnueabi/sysroot/usr/include/sys/ucontext.h:25,
                 from /home/naourr/work/instance-0/output-1/per-package/babeltrace2/host/arm-buildroot-linux-uclibcgnueabi/sysroot/usr/include/signal.h:329,
                 from /home/naourr/work/instance-0/output-1/per-package/babeltrace2/host/bin/../arm-buildroot-linux-uclibcgnueabi/sysroot/usr/include/glib-2.0/glib/gbacktrace.h:36,
                 from /home/naourr/work/instance-0/output-1/per-package/babeltrace2/host/bin/../arm-buildroot-linux-uclibcgnueabi/sysroot/usr/include/glib-2.0/glib.h:34,
                 from ../../src/common/assert.h:28,
                 from ../../src/lib/object.h:28,
                 from test_trace_ir_ref.c:25:
/home/naourr/work/instance-0/output-1/per-package/babeltrace2/host/arm-buildroot-linux-uclibcgnueabi/sysroot/usr/include/sys/user.h:48:8: note: originally defined here
 struct user
        ^~~~

Fixes:
 - http://autobuild.buildroot.org/results/e4229e2b9c892b419a9d2eaa6929b80ea62dd130

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Change-Id: I1a9d912545f781d61d6e3c62b31998285d2a237c
Reviewed-on: https://review.lttng.org/c/babeltrace/+/4522
CI-Build: Simon Marchi <simon.marchi@efficios.com>
Tested-by: jenkins <jenkins@lttng.org>
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>