Fix breakpad test app build under Linux
Fix breakpad test app build under Linux

Add missing pthreads library to build. This is required
for pthread_once() in the GUID allocator.

MD-20241
test: none

spthaolt/breakpad,Mendeley/breakpad,spthaolt/breakpad,sinuos/breakpad,spthaolt/breakpad,gvaf/breakpad,Mendeley/breakpad,spthaolt/breakpad,gvaf/breakpad,gvaf/breakpad,gvaf/breakpad,sinuos/breakpad,sinuos/breakpad,sinuos/breakpad,Mendeley/breakpad,Mendeley/breakpad