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)) {
          ^~~~~~