Make widget-control.c fails on gcc 16.

[ahysing]

TLDR; put a prototype for finish(1) at the top of widget-control.c

$ make
./make-widget
make[1]: Entering directory `C:/Users/AHysing/code/sdr-widget/Release'
make[1]: Nothing to be done for `all'.
make[1]: Leaving directory `C:/Users/AHysing/code/sdr-widget/Release'
gcc -o widget-control widget-control.c -lusb-1.0
widget-control.c: In function 'setup':
widget-control.c:251:22: error: implicit declaration of function 'finish' [-Wimplicit-function-declaration]
  251 |                 exit(finish(1));
      |                      ^~~~~~
make: *** [widget-control] Error 1

Running on windows 11

$ gcc --version
gcc.exe (Rev5, Built by MSYS2 project) 16.1.0
Copyright (C) 2026 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

avr32-gcc --version
avr32-gcc (AVR_32_bit_GNU_Toolchain_3.4.1_348) 4.4.3
Copyright (C) 2010 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.