Tests/VSGNUFortran: Avoid C++ comment in C code (#13314)

Tests/VSGNUFortran/c_code/main.c

@@ -1,4 +1,4 @@
-#include <HelloWorldFCMangle.h> // created by FortranCInterface
+#include <HelloWorldFCMangle.h> /* created by FortranCInterface */
 extern void FC_hello(void);
 int main()
 {