adb: sysdeps: add support for joining threads.

Bug: http://b/27105824
Change-Id: I44e4edbb2a59565c35f1f3e6a6394ac258591f95
(cherry picked from commit 3b3e10d0465506abc7d9e07c0381396b3726f183)
diff --git a/adb/Android.mk b/adb/Android.mk
index d629223..baa4985 100644
--- a/adb/Android.mk
+++ b/adb/Android.mk
@@ -58,6 +58,7 @@
 LIBADB_TEST_SRCS := \
     adb_io_test.cpp \
     adb_utils_test.cpp \
+    sysdeps_test.cpp \
     transport_test.cpp \
 
 LIBADB_CFLAGS := \