fix: cast EOF to char

[TheOneWithTheBraid]

Fixes build on aarch64 :

ninja: Entering directory `build'
[1/2] Compiling C object xdg-desktop-portal-termfilechooser.p/src_filechooser_filechooser.c.o
FAILED: xdg-desktop-portal-termfilechooser.p/src_filechooser_filechooser.c.o
cc -Ixdg-desktop-portal-termfilechooser.p -I. -I.. -I../include -fdiagnostics-color=always -D_FILE_OFFSET_BITS=64 -Wall -Winvalid-pch -Wextra -Werror -std=c11 -O0 -g -Wno-missing-braces -Wno-missing-field-initializers -Wno-unused-parameter -D_POSIX_C_SOURCE=200809L '-DSYSCONFDIR="/usr/local/etc"' -DHAVE_LIBSYSTEMD=1 -MD -MQ xdg-desktop-portal-termfilechooser.p/src_filechooser_filechooser.c.o -MF xdg-desktop-portal-termfilechooser.p/src_filechooser_filechooser.c.o.d -o xdg-desktop-portal-termfilechooser.p/src_filechooser_filechooser.c.o -c ../src/filechooser/filechooser.c
../src/filechooser/filechooser.c: In function ‘exec_filechooser’:
../src/filechooser/filechooser.c:62:17: error: comparison is always true due to limited range of data type [-Werror=type-limits]
   62 |     } while (cr != EOF);
      |                 ^~
cc1: all warnings being treated as errors
ninja: build stopped: subcommand failed.

[N-R-K]

The proper fix is to change cr to int which is what getc() returns.

See https://c-faq.com/stdio/getcharc.html