hidapi.h: fix API prototype for gcc flag -Wstrict-prototypes

[borneoa]

With gcc 10.2.0 and compile flag '-Wstrict-prototypes' the new
APIs in hidapi.h trigger a compile warning:

../hidapi/hidapi.h:481:32: warning: function declaration isn’t a prototype [-Wstrict-prototypes]
481 | HID_API_EXPORT const struct hid_api_version* HID_API_CALL hid_version();
| ^~~~~~~~~~~~~~~
../hidapi/hidapi.h:491:3: warning: function declaration isn’t a prototype [-Wstrict-prototypes]
491 | HID_API_EXPORT const char* HID_API_CALL hid_version_str();
| ^~~~~~~~~~~~~~

To replicate:
./bootstrap
CFLAG=-Wstrict-prototypes ./configure
make

This is an issue for other projects, like OpenOCD, that use hidapi
and compile by default with '-Wstrict-prototypes -Werror'; the
warning above causes compile error while including hidapi.h.

Fix the prototypes by adding 'void' in the empty parameter list.

Signed-off-by: Antonio Borneo borneo.antonio@gmail.com

[z3ntu]

Can we enable all these warning flags in CI as well that we're fixing so we don't regress?

[borneoa]

Yes, it could be good to add more strict check during CI.
With OpenOCD we use
-Wall -Wstrict-prototypes -Wformat-security -Wshadow -Wextra -Wno-unused-parameter -Wbad-function-cast -Wcast-align -Wredundant-decls -Werror
not sure if this set covers all needs of hidapi, but tested after this patch does not show any further warning.