[PATCH] uml: readd removed unistd.h inclusion

Readd this header (deleted in 60d339f6fe0831060600c62418b71a62ad26c281). A
warning is spit out here about undeclared getpgrp().

Signed-off-by: Paolo 'Blaisorblade' Giarrusso <blaisorblade@yahoo.it>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

arch/um/os-Linux/process.c

@@ -3,6 +3,7 @@
  * Licensed under the GPL
  */
 
+#include <unistd.h>
 #include <stdio.h>
 #include <errno.h>
 #include <signal.h>