package/strace: disable libmpers with musl toolchains

strace bundle some kernel headers to build libmpers, this mixes userspace
headers and kernel headers which break the build with musl.
The stddef.h from gcc is used instead of the one from musl.

Disable libmpers for musl toolchains.

Fixes:
http://autobuild.buildroot.net/results/6f6/6f63fa2537ff316ba259a20966faadcf5ab9deff



Signed-off-by: Romain Naour <romain.naour@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>