strace: disable (again) mpers for musl

strace: disable (again) mpers for musl

It turns out that mpers support in strace still does not play nicely
with musl libc. As explained in commit 7892a778b (package/strace:
disable libmpers with musl toolchains) headers mixup causes gcc header
to be included, instead of the musl one, resulting in conflicting types.
Commit 1088372941 (strace: bump to version 4.21) incorrectly enabled
mpers for musl. Revert that.

Fixes:
http://autobuild.buildroot.net/results/345/3452419498c074ca66f36f0f87263fa10662ac86/

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