[3.12] Add `extern "C"` around `PyTraceMalloc_` functions. (#127772) (#127817)

Add `extern "C"` around `PyTraceMalloc_` functions. (#127772)

Pretty much everything else exported by Python.h has an extern "C"
annotation, yet this header appears to be missing one.

(cherry picked from commit 2cdeb61b57e638ae46a04386330a12abe9cddf2c)

Co-authored-by: Peter Hawkins <hawkinsp@cs.stanford.edu>