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)) {
          ^~~~~~
17 jobs for kmssink-build-error in 51 minutes and 38 seconds (queued for 4 seconds)
Status Job ID Name Coverage
  Preparation
passed #425360
gst indent

00:00:31

passed #425359
manifest

00:00:37

 
  Build
passed #425366
build cerbero android universal

00:11:51

passed #425367
build cerbero cross win32

00:04:04

passed #425368
build cerbero cross win64

00:07:33

passed #425365
build cerbero fedora x86_64

00:08:07

manual #425370
gst-ios-12.2 allowed to fail manual
build cerbero ios universal
passed #425369
gst-macos-10.14
build cerbero macos x86_64

00:06:58

passed #425361
build fedora x86_64

00:07:52

passed #425362
build nodebug fedora x86_64

00:06:02

passed #425363
build static fedora x86_64

00:09:58

passed #425364
build static nodebug fedora x86_64

00:07:06

 
  Test
passed #425371
check fedora

00:03:39

passed #425372
integration testsuites fedora

00:28:18

passed #425373
valgrind bad

00:09:09

 
  Integrate
passed #425374
android universal examples

00:08:51

passed #425375
documentation

00:05:43