checkpatch: fix stringification macro defect

Fix checkpatch misreporting defect with stringification macros

ERROR: Macros with complex values should be enclosed in parenthesis
  #27: FILE: arch/arm/include/asm/kgdb.h:41:
  +#define ___to_string(X) #X

Signed-off-by: Joe Perches <joe@perches.com>
Reported-by: Vincent Stehlé <v-stehle@ti.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>