testing/powder-toy: fix ftbfs

Fix a ftbfs error with missing header include (fixed upstream) while rebuilding against jsoncpp 1.9.6.

Upgrading to 98.2.365 failed with an ../src/common/platform/stacktrace/Execinfo.cpp:3:10: fatal error: execinfo.h: No such file or directory error. execinfo.h is a glibc-specific header. It seems to be related to this commit that introduced stack tracing and should probably be reported to upstream.