Include common.h before any system headers.

This should fix some problems building on AIX.
check_nt.c - Changed 'Mb' to 'MB' in MEMUSE.
Merge pull request #1264 from waja/check_nt_units

check_nt.c - Changed 'Mb' to 'MB' in MEMUSE.
Merge pull request #1262 from waja/fix_aix

Include common.h before any system headers.