Makefile: Add new 'sip' testsuite to list of testsuites

Change-Id: Ie7cc41a40fb8706e9b0752be6e7c1f34dc692d87
diff --git a/Makefile b/Makefile
index aeca0e1..fcd6601 100644
--- a/Makefile
+++ b/Makefile
@@ -1,4 +1,4 @@
-SUBDIRS=bsc bsc-nat bts ggsn_tests hlr lapdm mgw msc pcu selftest sgsn sysinfo
+SUBDIRS=bsc bsc-nat bts ggsn_tests hlr lapdm mgw msc pcu selftest sgsn sip sysinfo
 
 PARALLEL_MAKE ?= -j8