Integrate openpower-libhei with openpower-hw-diags

Pull down openpower-libhei as a subproject and build the isolator
support into openpower-hw-diags.

Signed-off-by: Ben Tyner <ben.tyner@ibm.com>
Change-Id: I6eeacc802363fec6f0f113e75eb268a8c1774b7c
diff --git a/meson.build b/meson.build
index 9dc0caa..139d66f 100644
--- a/meson.build
+++ b/meson.build
@@ -3,9 +3,13 @@
         default_options: [
           'warning_level=3',
           'werror=true',
-          'cpp_std=c++17'
+          'cpp_std=c++17',
+          'cpp_args=-Wno-unused-parameter'
         ])
 
+# libhei is available as a subproject
+subproject('libhei')
+
 subdir('src')
 subdir('attn')