testsuite: fix hs_try_putmvar002 for targets without pthread.h

hs_try_putmvar002 includes pthread.h and doesn't work on targets
without this header (e.g. wasm32). It doesn't need to include this
header at all. This was previously unnoticed by wasm CI, though recent
toolchain upgrade brought in upstream changes that completely removes
pthread.h in the single-threaded wasm32-wasi sysroot, therefore we
need to handle that change.

testsuite/tests/concurrent/should_run/hs_try_putmvar002_c.c

 #include "HsFFI.h"
 #include <unistd.h>
-#include <pthread.h>
 #include "hs_try_putmvar002_stub.h"
 
 void externalPutMVar(HsStablePtr mvar, HsInt cap)