unicore32: EXPORT_SYMBOL needs export.h

Include export.h to fix below warning:

  CC      arch/unicore32/mm/init.o
arch/unicore32/mm/init.c:227: warning: data definition has no type or storage class
arch/unicore32/mm/init.c:227: warning: type defaults to 'int' in declaration of 'EXPORT_SYMBOL'
arch/unicore32/mm/init.c:227: warning: parameter names (without types) in function declaration

Signed-off-by: Axel Lin <axel.lin@gmail.com>
Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>