libxc: Reorder functions in xc_misc.c to avoid weak symbol problem

libxc: Reorder functions in xc_misc.c to avoid weak symbol problem

Using a function, and then declaring it weak later, has undefined
behaviour:
  cc1: warnings being treated as errors
  xc_misc.c:388: error: weak declaration of 'xc_map_foreign_bulk'
  after first use results in unspecified behavior

So swap the functions xc_map_foreign_pages and xc_map_foreign_bulk.

Signed-off-by: Ian Jackson <ian.jackson@eu.citrix.com>