headers_install: fix __packed in exported kernel headers

checkpatch.pl warns about using __attribute__((packed)) in kernel
headers: "__packed is preferred over __attribute__((packed))". If one
follows that advice it could cause problems in the exported header
files, because the outside world doesn't know about this shortcut.

For example busybox will fail to compile:
 CC      miscutils/ubi_attach_detach.o
 In file included from miscutils/ubi_attach_detach.c:27:0:
 /usr/include/mtd/ubi-user.h:330:3: error: conflicting types for ‘__packed’
 /usr/include/mtd/ubi-user.h:314:3: note: previous declaration of ‘__packed’ was here
...

Fix the problem by substituting __packed with __attribute__((packed)) in
the header_install.pl script.

Cc: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>
CC: Joe Perches <joe@perches.com>
Signed-off-by: Markus Trippelsdorf <markus@trippelsdorf.de>
Signed-off-by: Michal Marek <mmarek@suse.cz>