Don't depend on set being transitively included.

git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@183936 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/source/Host/common/FileSpec.cpp b/source/Host/common/FileSpec.cpp
index ac5b072..08d626e 100644
--- a/source/Host/common/FileSpec.cpp
+++ b/source/Host/common/FileSpec.cpp
@@ -12,6 +12,7 @@
 #include <fcntl.h>
 #include <libgen.h>
 #include <sys/stat.h>
+#include <set>
 #include <string.h>
 #include <fstream>