Fix missing 'fd_set' for musl

--- a/src/FDM/JSBSim/input_output/FGfdmSocket.cpp	2025-03-03 21:53:54.063997339 +0100
+++ b/src/FDM/JSBSim/input_output/FGfdmSocket.cpp	2025-03-03 21:55:29.808708089 +0100
@@ -47,6 +47,7 @@ INCLUDES
 #include <unistd.h>
 #include <fcntl.h>
 #else
+#include <sys/select.h>
 #include <fcntl.h>
 #include <unistd.h>
 #endif