toolchain: like glibc, musl always provides SSP support

Make sure BR2_TOOLCHAIN_USES_MUSL selects BR2_TOOLCHAIN_HAS_SSP since
musl always provides SSP support (like glibc).

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Acked-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>