kernel: enable CONFIG_ADVISE_SYSCALLS

Without this, posix_[fm]advise does not work. This causes issues with
btrfs-progs, which uses fadvise to drop caches.

Signed-off-by: Rosen Penev <[email protected]>

target/linux/generic/config-4.9

@@ -90,7 +90,7 @@ CONFIG_32BIT=y
 # CONFIG_ADM6996_PHY is not set
 # CONFIG_ADM8211 is not set
 # CONFIG_ADT7316 is not set
-# CONFIG_ADVISE_SYSCALLS is not set
+CONFIG_ADVISE_SYSCALLS=y
 # CONFIG_ADXRS450 is not set
 CONFIG_AEABI=y
 # CONFIG_AFE4403 is not set