package/libunwind: fix libucontext handling

Commit 6ea2a27f9047696ce1c463ad55fb4a78df136954 forgot to add -lucontext
to LIBS resulting in the following build failure with zeromq:

/home/buildroot/instance-0/output-1/host/opt/ext-toolchain/bin/../lib/gcc/i586-buildroot-linux-musl/9.3.0/../../../../i586-buildroot-linux-musl/bin/ld: /home/buildroot/instance-0/output-1/host/i586-buildroot-linux-musl/sysroot/usr/lib32/libunwind.so.8: undefined reference to `setcontext'

Fixes: 6ea2a27f9047696ce1c463ad55fb4a78df136954
 - http://autobuild.buildroot.org/results/893defe1588b2ca03c115b59b47be3f4aed438fb

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Arnout Vandecappelle <arnout@mind.be>

package/libunwind/libunwind.mk

@@ -17,6 +17,7 @@ LIBUNWIND_CONF_OPTS = \
 
 ifeq ($(BR2_PACKAGE_LIBUCONTEXT),y)
 LIBUNWIND_DEPENDENCIES += libucontext
+LIBUNWIND_CONF_OPTS += LIBS=-lucontext
 endif
 
 $(eval $(autotools-package))