Fix the following warning:
arch/sparc/vdso/vma.c:246:12: warning: no previous prototype for ‘init_vdso_image’

init_vdso_image has no users outside vma.c, make it static.
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Cc: Andreas Larsson <andreas@gaisler.com>
Cc: David S. Miller <davem@davemloft.net>
Reviewed-by: Andreas Larsson <andreas@gaisler.com>
Tested-by: Andreas Larsson <andreas@gaisler.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Link: https://lore.kernel.org/r/20240330-sparc64-warnings-v1-1-37201023ee2f@ravnborg.orgSigned-off-by: Andreas Larsson <andreas@gaisler.com>