Use the SimGear default notation.

simgear/misc/sgstream.cxx

@@ -21,11 +21,13 @@
 //
 // $Id$
 
+#include <simgear/compiler.h>
+#include STL_STRING 
+
 #include <ctype.h> // isspace()
 
 #ifdef SG_HAVE_STD_INCLUDES
 # include <cerrno>
-# include <string>
 #else
 # include <errno.h>
 #endif