install headers from runtime/backend

Looks like these are all public and we simply missed installing them.


ghstack-source-id: 6663892
ghstack-comment-id: 3207551477
Pull-Request: #13558

Author: swolchok

CMakeLists.txt

@@ -480,6 +480,13 @@ add_subdirectory(${CMAKE_CURRENT_SOURCE_DIR}/configurations)
 
 # Install `executorch` library as well as `executorch-config.cmake` under
 # ${CMAKE_INSTALL_PREFIX}/
+install(
+  DIRECTORY runtime/backend/
+  DESTINATION ${CMAKE_INSTALL_INCLUDEDIR}/executorch/runtime/backend
+  FILES_MATCHING
+  PATTERN "*.h"
+  PATTERN "test" EXCLUDE
+)
 install(
   DIRECTORY runtime/core/
   DESTINATION ${CMAKE_INSTALL_INCLUDEDIR}/executorch/runtime/core