Fix multiple definition of prefs compile error with GCC-10

[kenhys]

This commit fixes the following error:

libtool: link: gcc -pthread -I/usr/include/gtk-3.0 -I/usr/include/at-spi2-atk/2.0 -I/usr/include/at-spi-2.0 -I/usr/include/dbus-1.0 -I/usr/lib/x86_64-linux-gnu/dbus-1.0/include -I/usr/include/gtk-3.0 -I/usr/include/gio-unix-2.0 -I/usr/include/cairo -I/usr/include/pango-1.0 -I/usr/include/fribidi -I/usr/include/harfbuzz -I/usr/include/atk-1.0 -I/usr/include/cairo -I/usr/include/pixman-1 -I/usr/include/uuid -I/usr/include/freetype2 -I/usr/include/libpng16 -I/usr/include/gdk-pixbuf-2.0 -I/usr/include/libmount -I/usr/include/blkid -I/usr/include/glib-2.0 -I/usr/lib/x86_64-linux-gnu/glib-2.0/include -g -O2 -fdebug-prefix-map=/workspace/galculator-2.1.4=. -fstack-protector-strong -Wformat -Werror=format-security -Wall -Wl,-z -Wl,relro -o galculator galculator-main.o galculator-math_functions.o galculator-display.o galculator-general_functions.o galculator-calc_basic.o galculator-config_file.o galculator-callbacks.o galculator-ui.o galculator-flex_parser.o -Wl,--export-dynamic -Wl,--as-needed -lgtk-3 -lgdk-3 -lpangocairo-1.0 -lpango-1.0 -lharfbuzz -latk-1.0 -lcairo-gobject -lcairo -lgdk_pixbuf-2.0 -lgio-2.0 -lgobject-2.0 -lglib-2.0 -lm -lquadmath -pthread
/usr/bin/ld: galculator-config_file.o:./src/config_file.c:42: multiple definition of `prefs'; galculator-main.o:./src/main.c:52: first defined here
collect2: error: ld returned 1 exit status

[ib]

It's already declared extern in galculator.h (and defined in config_file.c).

[r-moeritz]

I can confirm the code doesn't build without this fix on GCC 13.