buildroot

musl: add more kernel headers guards

Kernel version 4.15 (not 4.16 as the musl commit log claims) allows
disabling of more parts of the kernel headers definitions. Add upstream
musl patch that defines the relevant macros. This solves issues of
networking related symbols redefinition in kernel headers that cause
headers conflicts. With that in place a subsequent commit will limit the
musl/kernel headers conflict avoidance workaround in Buildroot to kernel
headers older than 4.15. This workaround has been introduced in commit
196932cd91 (toolchain: workaround musl/kernel headers conflict).

Cc: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>