gcc-cross-canadian.inc: Fix buildpaths error for pthread.h

gcc-cross-canadian.inc: Fix buildpaths error for pthread.h

Replace the hardcoded path with /not/exist as used for other
options[--with-sysroot] to ensure pthread.h does not contain
hardocded references to TMPDIR:

ERROR: gcc-cross-canadian-x86-64-13.3.0-r0 do_package_qa: QA Issue:
File /usr/local/oe-sdk-hardcoded-buildpath/sysroots/x86_64-pokysdk-linux/
usr/lib/x86_64-poky-linux/gcc/x86_64-poky-linux/13.3.0/include-fixed/
pthread.h in package gcc-cross-canadian-x86-64 contains reference to
TMPDIR [buildpaths]

Signed-off-by: Sana Kazi <sanakazi720@gmail.com>
Signed-off-by: Steve Sakoman <steve@sakoman.com>