Fix build with boost >= 1.83

Index: src/cpp/core/system/PosixProcess.cpp
--- src/cpp/core/system/PosixProcess.cpp.orig
+++ src/cpp/core/system/PosixProcess.cpp
@@ -23,6 +23,8 @@
 
 #include <core/system/PosixSystem.hpp>
 
+#include <set>
+
 namespace rstudio {
 namespace core {
 namespace system {
