Remove openpower-libhei subproject dependency

Statically link to openpower-libhei rather than building it as a
subproject.

Signed-off-by: Ben Tyner <ben.tyner@ibm.com>
Change-Id: Ifc1be912ef484c0c7a1157ac0c9b90a62e03d9c7
diff --git a/meson.build b/meson.build
index 32f2201..e4ef613 100644
--- a/meson.build
+++ b/meson.build
@@ -7,8 +7,7 @@
           'cpp_args=-Wno-unused-parameter'
         ])
 
-# libhei is available as a subproject
-subproject('libhei')
+libhei_dep = dependency('hei', required : true)
 
 incdir = include_directories('.')