toolchain: limit musl workaround to kernel headers 3.12+

toolchain: limit musl workaround to kernel headers 3.12+

The libc-compat.h first appeared in kernel version 3.12. Trying to build a
musl toolchain using earlier headers leads to the following failure:

/bin/sed: can't read .../output/host/usr/arm-buildroot-linux-musleabi/sysroot/usr/include/linux/libc-compat.h: No such file or directory
package/pkg-generic.mk:266: recipe for target '.../output/build/toolchain/.stamp_staging_installed' failed

Don't apply the sed patch to older headers.

Reported-by: Florent Jacquet <florent.jacquet@free-electrons.com>
Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>