binfmt_flat: move the MAX_SHARED_LIBS definition to binfmt_flat.c

MAX_SHARED_LIBS is an implementation detail of the kernel loader,
and should be kept away from the file format definition.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Greg Ungerer <gerg@linux-m68k.org>