Revert "build-sys: compile procfs and sysfs code in Linux only"

This reverts commit f30b16edb4.

It seems better to use original solution and depend on HAVE_DIRFD
and HAVE_OPENAT, because libblkid and another tools are compiled
on non-Linux targets too.

lib/Makemodule.am

@@ -56,12 +56,10 @@ endif
 if HAVE_OPENAT
 if HAVE_DIRFD
 libcommon_la_SOURCES += lib/path.c
-if LINUX
 libcommon_la_SOURCES += lib/sysfs.c
 libcommon_la_SOURCES += lib/procfs.c
 endif
 endif
-endif
 
 EXTRA_LTLIBRARIES += libtcolors.la
 libtcolors_la_CFLAGS = $(AM_CFLAGS)