include/linux/eventfd.h: include linux/errno.h

The new gasket staging driver ran into a randconfig build failure when
CONFIG_EVENTFD is disabled:

  In file included from drivers/staging/gasket/gasket_interrupt.h:11,
                   from drivers/staging/gasket/gasket_interrupt.c:4:
  include/linux/eventfd.h: In function 'eventfd_ctx_fdget':
  include/linux/eventfd.h:51:9: error: implicit declaration of function 'ERR_PTR' [-Werror=implicit-function-declaration]

I can't see anything wrong with including eventfd.h before err.h, so the
easiest fix is to make it possible to do this by including the file
where it is needed.

Link: http://lkml.kernel.org/r/[email protected]
Fixes: 9a69f50 ("drivers/staging: Gasket driver framework + Apex driver")
Signed-off-by: Arnd Bergmann <[email protected]>
Cc: Eric Biggers <[email protected]>
Cc: Al Viro <[email protected]>
Cc: Greg Kroah-Hartman <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

include/linux/eventfd.h

@@ -11,6 +11,7 @@
 
 #include <linux/fcntl.h>
 #include <linux/wait.h>
+#include <linux/err.h>
 
 /*
  * CAREFUL: Check include/uapi/asm-generic/fcntl.h when defining