Libgpod code Merge Request #2: Make int arguments explicit (open)

Building using clang with the same default strictness as upcoming clang-16 fails with:

generic-callout.c:667:49: error: parameter 'usb_device_number' was not declared, defaults to 'int'; ISO C99 and later do not support implicit int [-Werror,-Wimplicit-int]
static char *get_info_from_usb (usb_bus_number, usb_device_number)
                                                ^
generic-callout.c:667:33: error: parameter 'usb_bus_number' was not declared, defaults to 'int'; ISO C99 and later do not support implicit int [-Werror,-Wimplicit-int]
static char *get_info_from_usb (usb_bus_number, usb_device_number)

Making the offending arguments explicitly gint (aka int) fixes the build.