Export ion_4.12.h and ion_4.19.h

Also, clean-up duplicate copies of the header files.

Bug: 140916230
Test: make, ./ion-unit-tests
Change-Id: Iab06ba622ab1d50e902ae20411bae73d0b00bbc7
diff --git a/libion/ion.c b/libion/ion.c
index 5141ea8..07b4caf 100644
--- a/libion/ion.c
+++ b/libion/ion.c
@@ -31,7 +31,7 @@
 #include <unistd.h>
 
 #include <ion/ion.h>
-#include "ion_4.19.h"
+#include <linux/ion_4.19.h>
 
 #include <log/log.h>