Require framework directory to exist when building

The framework directory will be provided by a submodule.

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
diff --git a/CMakeLists.txt b/CMakeLists.txt
index 5585c78..154c84a 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -277,6 +277,8 @@
     set(CMAKE_INSTALL_LIBDIR "${LIB_INSTALL_DIR}")
 endif()
 
+add_subdirectory(framework)
+
 add_subdirectory(include)
 
 add_subdirectory(3rdparty)