util: Fix linux build failure when SYS_kcmp isn't publicly accessible
This comes up specifically on older RHEL6 systems, which are still relatively common in HPC.
See https://github.com/spack/spack/issues/31866 :
253 ../src/util/os_file.c: In function 'os_same_file_description':
>> 254 ../src/util/os_file.c:211:19: error: 'SYS_kcmp' undeclared (first use in this function)
255 211 | return syscall(SYS_kcmp, pid, pid, KCMP_FILE, fd1, fd2);
256 | ^~~~~~~~
257 ../src/util/os_file.c:211:19: note: each undeclared identifier is reported only once for each function it appears in
>> 258 ../src/util/os_file.c:212:1: error: control reaches end of non-void function [-Werror=return-type]
259 212 | }
260 | ^
261 cc1: some warnings being treated as errors
Edited by Chuck Atkins