[flang][Runtime] Use proper prototypes in Fortran_main. NFCI

[rovka]

Summary

This is compiled as C code, so it's a good idea to be explicit about the
prototype. Clang complains about this when -Wstrict-prototypes is used.

[jeanPerier]

LGTM

[awarzynski]

Makes sense to me, thanks!

flang/include/flang/Runtime/main.h

 //===-- include/flang/Runtime/main.h ----------------------------*- C++ -*-===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 
 #ifndef FORTRAN_RUNTIME_MAIN_H_
 #define FORTRAN_RUNTIME_MAIN_H_
 
 #include "flang/Runtime/c-or-cpp.h"
 #include "flang/Runtime/entry-names.h"
 
 FORTRAN_EXTERN_C_BEGIN
 void RTNAME(ProgramStart)(int, const char *[], const char *[]);
-void RTNAME(ByteswapOption)(); // -byteswapio
+void RTNAME(ByteswapOption)(void); // -byteswapio
 FORTRAN_EXTERN_C_END
 
 #endif // FORTRAN_RUNTIME_MAIN_H_

flang/runtime/FortranMain/Fortran_main.c

 //===-- runtime/FortranMain/Fortran_main.c --------------------------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 
 #include "flang/Runtime/main.h"
 #include "flang/Runtime/stop.h"
 
 /* main entry into PROGRAM */
-void _QQmain();
+void _QQmain(void);
 
 /* C main stub */
 int main(int argc, const char *argv[], const char *envp[]) {
   RTNAME(ProgramStart)(argc, argv, envp);
   _QQmain();
   RTNAME(ProgramEndStatement)();
   return 0;
 }