cmake: install missing runtime/backend/ headers

The backend interface headers (backend_execution_context.h,
backend_init_context.h, interface.h, etc.) were not installed,
preventing out-of-tree consumers from using the backend API.

Co-authored-by: Claude <noreply@anthropic.com>

Author: tomeuv

CMakeLists.txt

@@ -596,6 +596,12 @@ install(
   FILES "${CMAKE_CURRENT_BINARY_DIR}/include/executorch/runtime/core/version.h"
   DESTINATION ${CMAKE_INSTALL_INCLUDEDIR}/executorch/runtime/core
 )
+install(
+  DIRECTORY runtime/backend/
+  DESTINATION ${CMAKE_INSTALL_INCLUDEDIR}/executorch/runtime/backend
+  FILES_MATCHING
+  PATTERN "*.h"
+)
 install(
   DIRECTORY runtime/executor/
   DESTINATION ${CMAKE_INSTALL_INCLUDEDIR}/executorch/runtime/executor