Add missing fs.h includes

The inclusion of this header should not depend on the inclusion of other
headers that include fs.h themselves.

src/ipc/process.h

@@ -5,6 +5,8 @@
 #ifndef BITCOIN_IPC_PROCESS_H
 #define BITCOIN_IPC_PROCESS_H
 
+#include <fs.h>
+
 #include <memory>
 #include <string>