src/pmdas/gfs2/pmda.c: add <sys/sysmacros.h> include
src/pmdas/gfs2/pmda.c: add <sys/sysmacros.h> include

According to gcc on Ubuntu 17.10 the definitions of makedev(), major()
and minor() are all in the process of migrating to this header.

adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp