BaseTools: Fix python3.8 SyntaxWarning

BaseTools: Fix python3.8 SyntaxWarning

Building with python3.8 shows a warning like:

SyntaxWarning: invalid escape sequence \(
  GuidName = re.compile("\(GUID=[-a-fA-F0-9]+")

It seems harmless, but it's easy enough to fix: mark the string as
raw with the 'r' prefix like is used elsewhere in the file

Signed-off-by: Cole Robinson <crobinso@redhat.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Bob Feng <bob.c.feng@intel.com>