toolchain-external: update musl-cross toolchain to 1.1.6

The 1.1.6 version of musl-cross fixes the two issues that had been
preventing versions after 1.1.1 being used by buildroot, namely:
- sysroot is enabled again
- kernel headers are included again

Signed-off-by: Will Wagner <will_wagner@carallon.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>