modpost: fix bogus 'exported twice' warnings.

Andi's change in e0f244c6 ("asmlinkage, module: Make ksymtab and
kcrctab symbols and __this_module __visible") make the crc appear
first in the symbol table.

modpost creates an entry when it sees the CRC, then when it sees the
actual symbol, it complains that it's seen it before.  The preloaded
flag already exists for the equivalent case where we loaded from
Module.symvers, so use that.

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Tested-by: The Awesome Power Of linux-next
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>