install header hei_util.hpp for external use

Signed-off-by: Zane Shelley <zshelle@us.ibm.com>
Change-Id: Iad4205bee94fe1298fdccf1a792bb99e1c33621b
diff --git a/meson.build b/meson.build
index 202f6d2..020bf73 100644
--- a/meson.build
+++ b/meson.build
@@ -47,6 +47,7 @@
     'src/hei_signature.hpp',
     'src/hei_types.hpp',
     'src/hei_user_interface.hpp',
+    'src/hei_util.hpp',
     subdir : 'libhei'
 )