Export local header files for libexif.

TEST=It still compiles.
BUG=26318506

Change-Id: I0b8ee76682fa1ce0d1d01d1f63cc2f67d13975c9
diff --git a/libexif/Android.mk b/libexif/Android.mk
index effa0c4..953c6b1 100644
--- a/libexif/Android.mk
+++ b/libexif/Android.mk
@@ -51,6 +51,9 @@
     pentax/mnote-pentax-entry.c \
     pentax/mnote-pentax-tag.c
 
+# Because all the include statements in the header files are in double layer
+# ("libexif/XXXX.h") style, we need to set the export root to the parent folder.
+LOCAL_EXPORT_C_INCLUDE_DIRS := $(LOCAL_PATH)/..
 
 LOCAL_MODULE := libexif