[PollyACC] Fix declaration/stub definition mismatch. NFC.

external.c defines stub functions that are never used because of how
Polly uses PPCG. Unfortunately, they are declared as functions without
return values or parameters which does not match their declarations.
Since they are never called, this was usually not a problem, but an LTO
build gets confused with differently declared functions, or in case of
pet_options_args, a global variable declaration that is defined as a
function

Resolve by including the declaring headers in external.c which forces
the declaration and definition to match at compile-time.

This fixes llvm.org/50021