checkpatch: add --strict test for concatenated string elements

Using a space between concatenated string elements is easier for a human
to read.

ie:
	"String"FOO"bar"

is easier to read as:

	"String" FOO "bar"

So suggest this style with a --strict command line option.

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