scope hei_bit_string.hpp for external use

This header will been needed by user applications.

Signed-off-by: Zane Shelley <zshelle@us.ibm.com>
Change-Id: I4eae57c63e3d314d87faed364a3c314c2300d20a
diff --git a/meson.build b/meson.build
index 3aebdd0..07b3dba 100644
--- a/meson.build
+++ b/meson.build
@@ -32,6 +32,7 @@
                                 install: true)
 
 install_headers(
+    'src/hei_bit_string.hpp',
     'src/hei_chip.hpp',
     'src/hei_isolation_data.hpp',
     'src/hei_main.hpp',