Add error prone dependency

The next update to desugar will require error_prone_annotations,
add the dependency to the makefile.

Test: m -j desugar
Change-Id: I2c8e6b3eb6650c2237ccefa09952f2836bc99060
diff --git a/Android.mk b/Android.mk
index 7250a34..5533013 100644
--- a/Android.mk
+++ b/Android.mk
@@ -8,6 +8,7 @@
     asm-5.2 \
     asm-commons-5.2 \
     asm-tree-5.2 \
+    error_prone_annotations-2.0.18 \
     guava-20.0 \
     jsr305-3.0.1 \
     dagger2-auto-value-host \