Move std::string str_validate to separate header

This is to fix compilation of squirrel on MacOS/clang, as squirrel
defines macros which clash with including <string>

src/saveload/saveload.cpp

@@ -41,6 +41,7 @@
 #include "../fileio_func.h"
 #include "../gamelog.h"
 #include "../string_func.h"
+#include "../string_func_extra.h"
 #include "../fios.h"
 #include "../error.h"