documentation: desktop build requires libcap-dev

[aromanovsky]

In file included from /home/lxe/Weekend/fsview/wrapper.h:10:0,
                 from /home/lxe/Weekend/fsview/conf/cmdarg.h:10,
                 from /home/lxe/Weekend/fsview/conf/cmdarg.cpp:7:
/home/lxe/Weekend/fsview/allsys.h:38:10: fatal error: sys/capability.h: No such file or directory
 #include <sys/capability.h>
          ^~~~~~~~~~~~~~~~~~
compilation terminated.
make[2]: *** [CMakeFiles/fsviewlib.dir/conf/cmdarg.cpp.o] Error 1
make[1]: *** [CMakeFiles/fsviewlib.dir/all] Error 2
make: *** [all] Error 2

The error goes away after installing libcap-dev (or libcap-devel in other distros).
The dependency needs to be documented.