Add a lock-free cache type for int->int functions

To be used for cmap and advance caching if desired.
diff --git a/src/Makefile.am b/src/Makefile.am
index fc4cb29..a50a199 100644
--- a/src/Makefile.am
+++ b/src/Makefile.am
@@ -19,6 +19,7 @@
 	hb-blob.cc \
 	hb-buffer-private.hh \
 	hb-buffer.cc \
+	hb-cache-private.hh \
 	hb-common.cc \
 	hb-fallback-shape-private.hh \
 	hb-fallback-shape.cc \