Enable LIBPLDMRESPONDER Flag with meson option

- The intent behind this commit is to add a compile
  flag when libpldmresponder meson option is enabled, so
  that the code under #ifdef LIBPLDMRESPONDER is added during
  the build time.

Signed-off-by: Manojkiran Eda <manojkiran.eda@gmail.com>
Change-Id: I730c4402ac0cc7b448c676f48df7b59c0c8683fc