Add missing wall_time.cc to Android.mk.

Change-Id: I1aee667a4d4a968a10f60a87376f6a38c1809454

jni/Android.mk

diff --git a/jni/Android.mk b/jni/Android.mk
index da08b08..7896d31 100644
--- a/jni/Android.mk
+++ b/jni/Android.mk
@@ -154,6 +154,7 @@
                    $(CERES_SRC_PATH)/types.cc \
                    $(CERES_SRC_PATH)/visibility_based_preconditioner.cc \
                    $(CERES_SRC_PATH)/visibility.cc \
+                   $(CERES_SRC_PATH)/wall_time.cc \
                    $(CERES_SRC_PATH)/generated/schur_eliminator_d_d_d.cc \
                    $(CERES_SRC_PATH)/generated/schur_eliminator_2_2_2.cc \
                    $(CERES_SRC_PATH)/generated/schur_eliminator_2_2_3.cc \