Set LOCAL_EXPORT_C_INCLUDE_DIRS.

This makes it so that users of this library don't need to
explictly add external/modp_b64 to LOCAL_C_INCLUDES.

Bug: 22353299
Change-Id: If9170824c0eec9182434053689892860a66c3514

Android.mk

diff --git a/Android.mk b/Android.mk
index 9892c72..689a98c 100644
--- a/Android.mk
+++ b/Android.mk
@@ -19,4 +19,5 @@
 LOCAL_SRC_FILES := modp_b64.cc
 LOCAL_CPP_EXTENSION := .cc
 LOCAL_CFLAGS := -Wall -Werror
+LOCAL_EXPORT_C_INCLUDE_DIRS := $(LOCAL_PATH)
 include $(BUILD_STATIC_LIBRARY)