Export libbinder headers from libkeystore_binder

libkeystore_binder's headers include libbinder headers and consumers
of those headers will require libbinder's include paths as well.

Bug: 27804373
Test: This compiles with a modified libbinder include path.

Change-Id: Id8aef2c91fe118d413115c204b2c81c559ad4134
diff --git a/keystore/Android.mk b/keystore/Android.mk
index 04ca220..059bfd4 100644
--- a/keystore/Android.mk
+++ b/keystore/Android.mk
@@ -110,6 +110,7 @@
 LOCAL_MODULE_TAGS := optional
 LOCAL_C_INCLUDES := $(LOCAL_PATH)/include $(call keystore_proto_include)
 LOCAL_EXPORT_C_INCLUDE_DIRS := $(LOCAL_PATH)/include
+LOCAL_EXPORT_SHARED_LIBRARY_HEADERS := libbinder
 LOCAL_ADDITIONAL_DEPENDENCIES := $(LOCAL_PATH)/Android.mk
 include $(BUILD_SHARED_LIBRARY)