iostat: fix build with musl

iostat: fix build with musl

musl does not provide access to the kernel defined HZ macro. Under Linux HZ is
always 100, so use that value.

Fixes:
http://autobuild.buildroot.net/results/dc9/dc98a572c7055cb66cf14648af750b7e0c8e136e/
http://autobuild.buildroot.net/results/fe3/fe3907858f90f766f1949be631427dbb079e08ed/
http://autobuild.buildroot.net/results/59e/59e7c8a3d94d06ba52ea5034fcabd9737a5314db/

and more.

Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>