Add `extern "C"` around `PyTraceMalloc_` functions. (GH-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>