md: fix deprecation warning with latest glibc

md: fix deprecation warning with latest glibc

md.c: In function ‘md_process’:
md.c:120:3: warning: ‘__makedev_from_sys_types’ is deprecated:
  In the GNU C Library, `makedev' 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 `makedev', include <sys/sysmacros.h> directly.
  If you did not intend to use a system-defined macro `makedev',
  you should #undef it after including <sys/types.h>.
  [-Wdeprecated-declarations]
   if (st.st_rdev != makedev (MD_MAJOR, minor))
   ^~
In file included from /usr/include/features.h:397:0,
                 from /usr/include/bits/libc-header-start.h:33,
                 from /usr/include/stdio.h:28,
                 from ./daemon/collectd.h:34,
                 from md.c:22: