fdkaac: plugin fails to build using fdk-aac 2.0.0

With the recently released fdk-aac 2.0.0 installed, compilation of the fdkaac plugin from git master fails with the following error:

gstfdkaacenc.c:74:8: error: 'MODE_2_1' undeclared here (not in a function); did you mean MODE_6_1'?
     3, MODE_2_1, {
        ^~~~~~~~
        MODE_6_1