Add missing include

[VladislavGrudinin]

Got following error when compiling with clang:

/var/tmp/portage/dev-python/pywlroots-0.16.5/work/pywlroots-0.16.5-python3_11/build/temp.linux-x86_64-cpython-311/wlroots._ffi.c:6368:3: error: call to undeclared function 'wlr_keyboard_notify_modifiers'; ISO C99 and later do not support implicit function declarations [-Wimplicit-function-declaration]
6368 | wlr_keyboard_notify_modifiers(x0, x1, x2, x3, x4);

This function was added in #123, but include was forgotten.

[m-col]

Thanks!

[flacjacket]

Thanks for catching this! How did you setup to use clang to build the cffi module? I'm curious if we could do something similar in the CI so this could be caught automatically.