kmssink: Fix implicit declaration build error
ffs() and strcmp() require string.h
gstkmssink.c:255:28: error: implicit declaration of function ‘ffs’ [-Werror=implicit-function-declaration]
crtc_id = res->crtcs[ffs (crtcs_for_connector) - 1];
^~~
gstkmssink.c:590:10: error: implicit declaration of function ‘strcmp’ [-Werror=implicit-function-declaration]
if (!strcmp (property->name, prop_name)) {
^~~~~~
Edited by Seungha Yang