fix section mismatch warning in lib/swiotlb.c

kbuild spits outs following warning on a
defconfig x86_64 build:
WARNING: swiotlb.o - Section mismatch: reference to .init.text:swiotlb_init from __ksymtab between '__ksymtab_swiotlb_init' (at offset 0xa0) and '__ksymtab_swiotlb_free_coherent'

This warning happens because the function swiotlb_init is marked __init and
EXPORT_SYMBOL().  A 'git grep swiotlb_init' showed no users in drivers/ so
remove the EXPORT_SYMBOL.

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Cc: Andi Kleen <ak@suse.de>
Cc: "Luck, Tony" <tony.luck@intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>