Compiler is not happy about one function prototype:

  CC      kernel/dma/swiotlb.o
  kernel/dma/swiotlb.c:275:1: warning: no previous prototype for ‘swiotlb_late_init_with_default_size’ [-Wmissing-prototypes]
  275 | swiotlb_late_init_with_default_size(size_t default_size)
      | ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Since it's used outside of the module, move its declaration to the header
from the user.
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>