checkpatch: add check for space after struct, union, and enum

Add spacing checks for struct, union, and enum definitions.  Check the
spacing after type and before the equals (=) and open brace ({).

Based on a patch by Joe Perches.

Cc: Joe Perches <joe@perches.com>
Signed-off-by: Andy Whitcroft <apw@canonical.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>