build: Correcting dependencies in [examples/tests/tests_boost]/Makefile.am so the code can be compiled on Linux

Change-Id: I755faa16f3c1eb5b3a8f89a46a67533ef7efb89c
diff --git a/examples/Makefile.am b/examples/Makefile.am
index 7712cb7..ee2b0e3 100644
--- a/examples/Makefile.am
+++ b/examples/Makefile.am
@@ -1,5 +1,5 @@
 
-LDADD = ../libndn-cpp.la
+LDADD = ../libndn-cpp.la @BOOST_SYSTEM_LIB@
 
 noinst_PROGRAMS=consumer producer