kconfig: fix missing "0x" prefix from S_HEX symbol in autoconf.h

The specialized printer for headers (espectially autoconf.h) is missing
fixup code for S_HEX symbol's "0x" prefix. As long as kconfig does not
warn for such missing prefix, this code is needed. Fix this.

In the same time, fix some nits in `header_print_symbol()'.

Cc: Randy Dunlap <rdunlap@xenotime.net>
Cc: Mauro Carvalho Chehab <mchehab@infradead.org>

Broken-by: Arnaud Lacombe <lacombar@gmail.com>
Reported-by: Randy Dunlap <rdunlap@xenotime.net>
Reported-by: Mauro Carvalho Chehab <mchehab@infradead.org>
Acked-by: Mauro Carvalho Chehab <mchehab@redhat.com>
Acked-by: Randy Dunlap <rdunlap@xenotime.net>
Signed-off-by: Arnaud Lacombe <lacombar@gmail.com>
Signed-off-by: Michal Marek <mmarek@suse.cz>