tests: Add '--duplicate-sources' to libgem5 SST build

Unfortunately the building of SST (in "ext/sst") depends on generated
header files. Therefore it adds the build directory to the include path.
For this to work the libgem5.so must be build with
"--duplicate-sources".

Change-Id: I5ed26a89c81402d421f5dfa110de7cf758f28694
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/70497
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>
2 files changed