Make istty_stdout definition static

[ltning]

Without this, compilation fails with

/usr/local/bin/ld: /tmp//cc1rco4B.o:(.bss+0x2080220): multiple definition of `istty_stdout'; /tmp//ccIaYcw6.o:(.bss+0x0): first defined here
/usr/local/bin/ld: /tmp//cc1rco4B.o:(.bss+0x2080221): multiple definition of `istty_stderr'; /tmp//ccIaYcw6.o:(.bss+0x1): first defined here

..at least on FreeBSD and GCC 12. With this change, it compiles and runs as prescribed.