package/libunwind: don't override dependencies

Don't override LIBUNWIND_DEPENDENCIES in a conditional

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
(cherry picked from commit 794821faa339c4b7910bb3f4e9299f887acc901e)
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

package/libunwind/libunwind.mk

@@ -16,7 +16,7 @@ LIBUNWIND_CONF_OPTS = \
 	$(if $(BR2_INSTALL_LIBSTDCPP),--enable-cxx-exceptions,--disable-cxx-exceptions)
 
 ifeq ($(BR2_PACKAGE_LIBATOMIC_OPS),y)
-LIBUNWIND_DEPENDENCIES = libatomic_ops
+LIBUNWIND_DEPENDENCIES += libatomic_ops
 endif
 
 $(eval $(autotools-package))