Fix minor CMake issue

Summary:
For CMake version < 3.19 we can't do `add_library(<name> INTERFACE <sources>)`. See https://cmake.org/cmake/help/v3.19/command/add_library.html#interface-libraries

Since the minimum requirement is 3.13 we better make it work for < 3.19.

Reviewed By: dbort

Differential Revision: D48084620

fbshipit-source-id: 262b8d7d0f6e5cef821bbeebc76f8d964a1de59d

Author: larryliu0820

CMakeLists.txt

@@ -134,7 +134,9 @@ add_custom_command(
   DEPENDS flatc ${_program_schema__srcs}
   COMMENT "Generating program_schema headers"
   VERBATIM)
-add_library(program_schema INTERFACE ${_program_schema__outputs})
+
+add_library(program_schema ${_program_schema__outputs})
+set_target_properties(program_schema PROPERTIES LINKER_LANGUAGE CXX)
 target_include_directories(
   program_schema
   INTERFACE ${_program_schema__include_dir}