LU-16510 build: check if CONFIG_FORTIFY_SOURCE is defined

LU-16510 build: check if CONFIG_FORTIFY_SOURCE is defined

The linux/fortify-string.h header file should not be
included while the kernel config option CONFIG_FORTIFY_SOURCE
is not defined.

Change-Id: I2e1905406e892b182f143d512a2d3722b141e52d
Fixes: 919b93b951d4 ("LU-16510 build: fortified memcpy from linux 6.1")
Signed-off-by: Jian Yu <yujian@whamcloud.com>
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/51973
Reviewed-by: Patrick Farrell <pfarrell@whamcloud.com>
Reviewed-by: Shaun Tancheff <shaun.tancheff@hpe.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>