Add covariance related files to the android build.

Change-Id: Ica8bf2b28bcbf6cde338f9ca3c64cd8a7d968a4c

jni/Android.mk

diff --git a/jni/Android.mk b/jni/Android.mk
index ec59f52..45b95b4 100644
--- a/jni/Android.mk
+++ b/jni/Android.mk
@@ -126,6 +126,8 @@
                    $(CERES_SRC_PATH)/conjugate_gradients_solver.cc \
                    $(CERES_SRC_PATH)/coordinate_descent_minimizer.cc \
                    $(CERES_SRC_PATH)/corrector.cc \
+                   $(CERES_SRC_PATH)/covariance.cc \
+                   $(CERES_SRC_PATH)/covariance_impl.cc \
                    $(CERES_SRC_PATH)/dense_normal_cholesky_solver.cc \
                    $(CERES_SRC_PATH)/dense_qr_solver.cc \
                    $(CERES_SRC_PATH)/dense_sparse_matrix.cc \