BaseTools: explicitly import email.header PatchCheck.py

BaseTools: explicitly import email.header PatchCheck.py

On Debian 10 (Buster), when running PatchCheck.py with python2, a
backtrace is printed, starting from:

  File "../edk2/BaseTools/Scripts/PatchCheck.py", line 595,
   in find_patch_pieces
    parts = email.header.decode_header(pmail.get('subject'))
  AttributeError: 'module' object has no attribute 'header'

When using python3, this backtrace does not appear.

Explicitly importing email.header resolves this for python2 and does not
appear to cause any issues with python3.

Cc: Bob Feng <bob.c.feng@intel.com>
Cc: Liming Gao <liming.gao@intel.com>
Signed-off-by: Leif Lindholm <leif@nuviainc.com>
Reviewed-by: Bob Feng <bob.c.feng@intel.com>