Blackfin: pnav: delete duplicate linux/export.h include

arch/blackfin/mach-bf537/boards/pnav10.c includes 'linux/export.h'
twice.  No need to include the file a second time in a #ifdef block
if already included at top.  Remove the duplicate.

Signed-off-by: Danny Kukawka <danny.kukawka@bisect.de>
Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Signed-off-by: Bob Liu <lliubbo@gmail.com>