package/babeltrace2: link with libatomic if needed

Fix build of babeltrace2 in version 2.0.3 with Bootlin SPARC uclibc
toolchain added with commit 1348c569d0cb7f67eca30f170b782aa8b51cc259

Fixes:
 - http://autobuild.buildroot.org/results/31770bf70f9ce4e3be8fb310d084b214820c6829

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>

package/babeltrace2/babeltrace2.mk

@@ -27,5 +27,9 @@ else
 BABELTRACE2_CONF_OPTS += --disable-debug-info
 endif
 
+ifeq ($(BR2_TOOLCHAIN_HAS_LIBATOMIC),y)
+BABELTRACE2_CONF_ENV += LIBS=-latomic
+endif
+
 $(eval $(autotools-package))
 $(eval $(host-autotools-package))