initramfs: Really fix build break on symbol-prefixed archs

Define the __initramfs_size variable using VMLINUX_SYMBOL() to take care
of symbol-prefixed architectures, for example, blackfin.

Signed-off-by: Hendrik Brueckner <brueckner@linux.vnet.ibm.com>
Cc: Mike Frysinger <vapier.adi@gmail.com>
Cc: Hendrik Brueckner <brueckner@linux.vnet.ibm.com>,
Cc: Michal Marek <mmarek@suse.cz>
Cc: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
[mmarek: leave out Makefile change, since d63f6d1b

 already takes care of the
 SYMBOL_PREFIX define]
Signed-off-by: Michal Marek <mmarek@suse.cz>