.python/SpellCheck: Add 'XIPFLAGS' to "words" section

.python/SpellCheck: Add 'XIPFLAGS' to "words" section

The EDKII Core CI reports spelling error for XIPFLAGS. The
XIPFLAGS are typically used to specify XIP options to the
compiler. e.g. GCC:*_*_*_CC_XIPFLAGS = -fno-jump-tables

Add 'XIPFLAGS' to "words" section in cspell.base.yaml file
to avoid spelling check error.

Signed-off-by: Sami Mujawar <sami.mujawar@arm.com>
Acked-by: Ard Biesheuvel <ard.biesheuvel@arm.com>
Acked-by: Laszlo Ersek <lersek@redhat.com>