Add missing header into config/checksrc/incorrect_mpi_abort_exitcode.c.

Some flavours of Clang (e.g. Apple Clang) compile with -Werror=implicit-function-declaration by default, which leads to the following error when compiling the test program:

incorrect_mpi_abort_exitcode.c:63:3: error: implicit declaration of function 'write' is invalid in C99 [-Werror,-Wimplicit-function-declaration]
  write(fd, exit_msg, sizeof (exit_msg));
  ^
incorrect_mpi_abort_exitcode.c:64:3: error: implicit declaration of function 'close' is invalid in C99 [-Werror,-Wimplicit-function-declaration]
  close(fd);
  ^
2 errors generated.

That in turn leads to a false negative result of the respective configure-time check.

This adds the missing header file.