Fix cmake compilation with dvdcss

The commit fixes the following error, trying to link libmythdvdnav.

/usr/bin/ld: ../../../../external/libmythdvdnav/libmythdvdnav-35.so.35.0: undefined reference to `dvdcss_open'
/usr/bin/ld: ../../../../external/libmythdvdnav/libmythdvdnav-35.so.35.0: undefined reference to `dvdcss_close'
/usr/bin/ld: ../../../../external/libmythdvdnav/libmythdvdnav-35.so.35.0: undefined reference to `dvdcss_open_stream'
/usr/bin/ld: ../../../../external/libmythdvdnav/libmythdvdnav-35.so.35.0: undefined reference to `dvdcss_read'
/usr/bin/ld: ../../../../external/libmythdvdnav/libmythdvdnav-35.so.35.0: undefined reference to `dvdcss_seek'
collect2: error: ld returned 1 exit status

mythtv/external/libmythdvdnav/CMakeLists.txt

@@ -122,6 +122,9 @@ target_link_libraries(mythdvdnav PUBLIC ${CMAKE_DL_LIBS})
 if(APPLE)
   target_link_libraries(mythdvdnav PRIVATE ${APPLE_CORE_FOUNDATION})
 endif()
+if(HAVE_DVDCSS_DVDCSS_H)
+  target_link_libraries(mythdvdnav PRIVATE dvdcss)
+endif()
 
 #
 # The MythTV customization to dvdnav adds references to the MythFile* functions,