package/strace: disable for riscv32

strace does not support riscv32 yet.

https://lists.strace.io/pipermail/strace-devel/2019-August/009068.html

Fixes:
http://autobuild.buildroot.net/results/912776cc1da1719806058516a2cc2a47c8dbad9b/



Cc: Mark Corbin <mark.corbin@embecosm.com>
Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>