Makefile: Add 'sgsn' sub-directory in list of projects

Change-Id: I2b9e5a28e60f06d05ac4ac2e3ed90c6bf72e03f2
diff --git a/Makefile b/Makefile
index 8999ec8..2fcc561 100644
--- a/Makefile
+++ b/Makefile
@@ -1,4 +1,4 @@
-SUBDIRS=bsc bsc-nat ggsn_tests gprs_gb lapdm mgw msc selftest sysinfo
+SUBDIRS=bsc bsc-nat ggsn_tests gprs_gb lapdm mgw msc selftest sgsn sysinfo
 
 PARALLEL_MAKE:=-j4