Work around issue with sysmacros.h and gcc7 / glibc 2.25.

Setting -Werror tricks the test into failing on the

"error: In the GNU C Library, "major" is defined by <sys/sysmacros.h>.
 For historical compatibility, it is  currently defined by <sys/types.h>
 as well, but we plan to remove this soon. To use "major", include
 <sys/sysmacros.h> directly. If you did not intend to use a system-defined
 macro "major", you should undefine it after including <sys/types.h>."

error. Since the normal Slurm build uses -Werror, this warning on including
both headers will then cause the build itself to fail.

Bug 3982.