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)) {
          ^~~~~~

sys/kms/gstkmssink.c

@@ -55,6 +55,7 @@
 #include <xf86drm.h>
 #include <xf86drmMode.h>
 #include <drm_fourcc.h>
+#include <string.h>
 
 #include "gstkmssink.h"
 #include "gstkmsutils.h"