Fix unit test build on Windows
Like Tom de Vries' earlier patch to fix the no-CXX_STD_THREAD case in
maint.c, this patch fixes a similar problem in
parallel-for-selftests.c. This fixes a build failure on Windows.
diff --git a/gdb/unittests/parallel-for-selftests.c b/gdb/unittests/parallel-for-selftests.c
index a2cd472..f149e72 100644
--- a/gdb/unittests/parallel-for-selftests.c
+++ b/gdb/unittests/parallel-for-selftests.c
@@ -20,10 +20,11 @@
#include "defs.h"
#include "gdbsupport/selftest.h"
#include "gdbsupport/parallel-for.h"
-#include "gdbsupport/thread-pool.h"
#if CXX_STD_THREAD
+#include "gdbsupport/thread-pool.h"
+
namespace selftests {
namespace parallel_for {
