package/stress-ng: link with -latomic when needed

Fix the following build failure raised since at least bump to version
0.15.04 in commit 00553ea186357fd3e2b3c89fa560e9711cc67472:

Fixes:
 - http://autobuild.buildroot.org/results/127ed4c110d99c6453a01ce221f628d40e566dc1

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>
(cherry picked from commit 42f25180233df459cd2bfbc5b9ebf8b95c6b60cb)
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

package/stress-ng/stress-ng.mk

@@ -20,6 +20,10 @@ ifeq ($(BR2_PACKAGE_KEYUTILS),y)
 STRESS_NG_DEPENDENCIES += keyutils
 endif
 
+ifeq ($(BR2_TOOLCHAIN_HAS_LIBATOMIC),y)
+STRESS_NG_MAKE_FLAGS += LDFLAGS_EXTRA=-latomic
+endif
+
 define STRESS_NG_BUILD_CMDS
 	$(TARGET_CONFIGURE_OPTS) $(MAKE) -C $(@D) $(STRESS_NG_MAKE_FLAGS)
 endef