[PSTL][NFC] Add missing #include

llvm-svn: 356917
GitOrigin-RevId: 524e5e901947d4c2fe56f7ed412d949ca453a7f6
diff --git a/include/pstl/internal/algorithm_impl.h b/include/pstl/internal/algorithm_impl.h
index c8e2e1b..99fc8c0 100644
--- a/include/pstl/internal/algorithm_impl.h
+++ b/include/pstl/internal/algorithm_impl.h
@@ -18,6 +18,7 @@
 
 #include "execution_impl.h"
 #include "memory_impl.h"
+#include "parallel_backend_utils.h"
 #include "unseq_backend_simd.h"
 
 #if __PSTL_USE_PAR_POLICIES