kernel/generic: add CONFIG_KCSAN to the 5.10 kconfig

Add the missing CONFIG_KCSAN (disabled). Found while making kernel_oldconfig on
an x86-64 subtarget.

Signed-off-by: Rui Salvaterra <rsalvaterra@gmail.com>

target/linux/generic/config-5.10

@@ -2818,6 +2818,7 @@ CONFIG_KALLSYMS_BASE_RELATIVE=y
 CONFIG_KASAN_STACK=1
 # CONFIG_KCMP is not set
 # CONFIG_KCOV is not set
+# CONFIG_KCSAN is not set
 # CONFIG_KERNEL_BZIP2 is not set
 # CONFIG_KERNEL_CAT is not set
 # CONFIG_KERNEL_GZIP is not set