bcm47xx: Fix build regression

Commit 0bc2d534708b ("rcu: Refactor #includes from include/linux/rcupdate.h")
caused a build regression in an MTD partition driver:

In file included from drivers/mtd/bcm47xxpart.c:12:0:
include/linux/bcm47xx_nvram.h: In function 'bcm47xx_nvram_init_from_mem':
include/linux/bcm47xx_nvram.h:27:10: error: 'ENOTSUPP' undeclared (first use in this function)

The rcupdate.h file has no particular need for linux/errno.h, so this
commit includes linux/errno.h into bcm47xx_nvram.h.

Fixes: 0bc2d534708b ("rcu: Refactor #includes from include/linux/rcupdate.h")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>