cc: fixes for Plan 9 build

<ctype.h> has been moved into <u.h>, specifically to be able to
drop it from these modules.

Will someone check platforms other than UBUNTU/386, please?

R=bsiegert, rsc
CC=golang-dev
https://golang.org/cl/4648078

src/cmd/cc/dpchk.c

@@ -29,7 +29,6 @@
 // THE SOFTWARE.
 
 #include	<u.h>
-#include	<ctype.h>
 #include	"cc.h"
 #include	"y.tab.h"
 

src/cmd/cc/lex.c

@@ -29,7 +29,6 @@
 // THE SOFTWARE.
 
 #include	<u.h>
-#include	<ctype.h>
 #include	"cc.h"
 #include	"y.tab.h"
 

src/cmd/cc/mac.c

@@ -29,7 +29,6 @@
 // THE SOFTWARE.
 
 #include	<u.h>
-#include	<ctype.h>
 #include	"cc.h"
 
 #include	"macbody"