libc: fix fropen/fwopen compile warnings

libc: fix fropen/fwopen compile warnings

This patch fixes warnings that appears when compiling:

#define fwopen(__cookie,__fn) funopen(__cookie, (int (*)())0, __fn,
(fpos_t (*)())0, (int (*)())0)
Expands to:

funopen(__null, (int (*)())0, &app_printf, (fpos_t (*)())0, (int
(*)())0)
argument of type "int (*)()" is incompatible with parameter of type
"int (*)(void *__cookie, char *__buf, int __n)"C/C++(167)
invalid conversion from 'fpos_t (*)()' {aka 'long int (*)()'} to
'fpos_t (*)(void*, fpos_t, int)' {aka 'long int (*)(void*,

Discussion is here:
https://github.com/espressif/arduino-esp32/issues/7407