libusb: make hid.c compile cleanly as C++

[Youw]

Problem

libusb/hid.c doesn't compile cleanly as C++, as reported in #671. Reproduced on current master with g++ -xc++ -Wall -Wextra -c libusb/hid.c:

libusb/hid.c:23: warning: "_GNU_SOURCE" redefined
libusb/hid.c:385:22: error: invalid conversion from 'void*' to 'wchar_t*'
libusb/hid.c:722:49: error: invalid conversion from 'void*' to 'hid_device_info*'
libusb/hid.c:1094:37: error: invalid conversion from 'void*' to 'hid_device*'
libusb/hid.c:1163:27: error: invalid conversion from 'void*' to 'hid_device*'
libusb/hid.c:1639:27: error: invalid conversion from 'void*' to 'hid_device*'
libusb/hid.c:2103:24: error: invalid conversion from 'void*' to 'char*'
libusb/hid.c:349:56: error: invalid conversion from 'int' to 'libusb_error'
libusb/hid.c:605:67: error: invalid conversion from 'int' to 'libusb_error'
... (13 such int → libusb_error sites total)

Three C-only patterns are involved:

1. Implicit void* to typed pointer conversions from malloc / calloc / pthread-style void *param (6 sites).
2. Implicit int to enum libusb_error conversions when storing libusb return codes (typed int) into the hidapi_error_ctx fields or passing them to register_libusb_error() (13 sites).
3. #define _GNU_SOURCE without an #ifndef guard — emits a "_GNU_SOURCE" redefined warning when the macro is also supplied via -D on the command line.

Change

Minimal fix, scoped to libusb/hid.c:

• Wrap #define _GNU_SOURCE in #ifndef _GNU_SOURCE.
• Change the internal hidapi_error_ctx struct's error_code / last_error_code_cache and the register_libusb_error() error parameter from enum libusb_error to plain int. The struct is internal (not exported), the stored values are already a mix of libusb error codes and a 1 sentinel from register_string_error(), and using int removes 13 call-site conversions in a single structural change.
• Add explicit C-style casts on the 6 malloc / calloc / void* sites. This matches the convention already in windows/hid.c and mac/hid.c.
• Drop a now-redundant (enum libusb_error) cast at libusb_get_device_list's error path.

Net diff: 17 insertions, 10 deletions in one file.

Verification

On WSL Ubuntu (gcc 11.4, libusb-1.0):

build	result
g++ -xc++ -Wall -Wextra -c libusb/hid.c	clean — was 19 errors + 1 warning
gcc -Wall -Wextra -c libusb/hid.c	clean — no change for the C build
full CMake build of the libusb backend (libhidapi-libusb.so, hidtest_libusb)	builds and links unchanged

Closes #671.

Drafted with Claude Code.