initramfs: Really fix build break on symbol-prefixed archs

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 <[email protected]>
Cc: Mike Frysinger <[email protected]>
Cc: Hendrik Brueckner <[email protected]>,
Cc: Michal Marek <[email protected]>
Cc: Sam Ravnborg <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
[mmarek: leave out Makefile change, since d63f6d1 already takes care of the
 SYMBOL_PREFIX define]
Signed-off-by: Michal Marek <[email protected]>