checkpatch.pl: add union and struct to the exceptions list

Here is a small code snippet, which will be complained about by
checkpatch.pl:

#define __STRUCT_KFIFO_COMMON(recsize, ptrtype) \
	union { \
		struct { \
			unsigned int	in; \
			unsigned int	out; \
		}; \
		char		rectype[recsize]; \
		ptrtype		*ptr; \
		const ptrtype	*ptr_const; \
	};

This construct is legal and safe, so checkpatch.pl should accept this.  It
should be also true for struct defined in a macro.

Add the `struct' and `union' keywords to the exceptions list of the
checkpatch.pl script, to prevent error message "Macros with multiple
statements should be enclosed in a do - while loop".  Otherwise it is not
possible to build a struct or union with a macro.

Signed-off-by: Stefani Seibold <stefani@seibold.net>
Cc: Andy Whitcroft <apw@shadowen.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>