arch/unicore32/mm/alignment.c: include "asm/pgtable.h" to avoid compiling error

Need include "asm/pgtable.h" to include "asm-generic/pgtable-nopmd.h",
so can let 'pmd_t' defined. The related error with allmodconfig:

    CC      arch/unicore32/mm/alignment.o
  In file included from arch/unicore32/mm/alignment.c:24:
  arch/unicore32/include/asm/tlbflush.h:135: error: expected .). before .*. token
  arch/unicore32/include/asm/tlbflush.h:154: error: expected .). before .*. token
  In file included from arch/unicore32/mm/alignment.c:27:
  arch/unicore32/mm/mm.h:15: error: expected .=., .,., .;., .sm. or ._attribute__. before .*. token
  arch/unicore32/mm/mm.h:20: error: expected .=., .,., .;., .sm. or ._attribute__. before .*. token
  arch/unicore32/mm/mm.h:25: error: expected .=., .,., .;., .sm. or ._attribute__. before .*. token
  make[1]: *** [arch/unicore32/mm/alignment.o] Error 1
  make: *** [arch/unicore32/mm] Error 2

Signed-off-by: Chen Gang <gang.chen.5i5j@gmail.com>
Acked-by: Xuetao Guan <gxt@mprc.pku.edu.cn>
Signed-off-by: Xuetao Guan <gxt@mprc.pku.edu.cn>