Update Linux kernel version number in tst-mman-consts.py to 5.2.

The tst-mman-consts.py test includes a kernel version number, to avoid
failures because of newly added constants in the kernel (if kernel
headers are newer than this version of glibc) or missing constants in
the kernel (if kernel headers are older than this version of glibc).
This patch updates it to 5.2 to reflect that the MAP_* constants in
glibc are still current as of that kernel version.

Tested with build-many-glibcs.py.

	* sysdeps/unix/sysv/linux/tst-mman-consts.py (main): Update Linux
	kernel version number to 5.2.