Fix Android.mk breakage

Change-Id: I01baba69999ae3bcca9f44ebb3a86232759999e2

jni/Android.mk

diff --git a/jni/Android.mk b/jni/Android.mk
index 7d5afd8..49cad30 100644
--- a/jni/Android.mk
+++ b/jni/Android.mk
@@ -161,7 +161,6 @@
                    $(CERES_SRC_PATH)/program.cc \
                    $(CERES_SRC_PATH)/residual_block.cc \
                    $(CERES_SRC_PATH)/residual_block_utils.cc \
-                   $(CERES_SRC_PATH)/runtime_numeric_diff_cost_function.cc \
                    $(CERES_SRC_PATH)/schur_complement_solver.cc \
                    $(CERES_SRC_PATH)/schur_eliminator.cc \
                    $(CERES_SRC_PATH)/schur_jacobi_preconditioner.cc \