kstack_erase: Fix missed export of renamed KSTACK_ERASE_CFLAGS

Certain targets disable kstack_erase by filtering out KSTACK_ERASE_CFLAGS
rather than adding DISABLE_KSTACK_ERASE. The renaming to kstack_erase
missed the CFLAGS export, which broke those build targets (e.g. x86
vdso32).

Fixes: 76261fc7 ("stackleak: Split KSTACK_ERASE_CFLAGS from GCC_PLUGINS_CFLAGS")
Signed-off-by: Kees Cook <kees@kernel.org>