debuggerd: fork the signal sender once.

Bug: http://b/27427439
Change-Id: I6294ff68a150bc9950a300264c31d2141307ac66
diff --git a/debuggerd/Android.mk b/debuggerd/Android.mk
index 9e4f1f7..6469db4 100644
--- a/debuggerd/Android.mk
+++ b/debuggerd/Android.mk
@@ -15,6 +15,7 @@
     debuggerd.cpp \
     elf_utils.cpp \
     getevent.cpp \
+    signal_sender.cpp \
     tombstone.cpp \
     utility.cpp \