src/libcgroup-internal.h: fix checkpatch.pl warnings

src/libcgroup-internal.h: fix checkpatch.pl warnings

Fix all of the warnings/errors reported by Linux Kernel's checkpatch.pl,
except SPDX_LICENSE_TAG, USE_NEGATIVE_ERRNO types.  It also introduces
reverse xmas tree local variable declarations and header file
reordering.

In summary, this patch fixes the following checkpatch.pl
recommendations:
total: 5 errors, 2 warnings, 438 lines checked

Signed-off-by: Kamalesh Babulal <kamalesh.babulal@oracle.com>
Signed-off-by: Tom Hromatka <tom.hromatka@oracle.com>