virt-viewer: fix SPICE support

* Add missing dependency on 'spice_protocol'

* Fix new build error which came now that ./configure enables SPICE support:

  building virt-viewer
    CCLD     virt-viewer
  /nix/store/b8qhjrwf8sf9ggkjxqqav7f1m6w83bh0-binutils-2.23.1/bin/ld: cannot find -lgdbm
  /nix/store/b8qhjrwf8sf9ggkjxqqav7f1m6w83bh0-binutils-2.23.1/bin/ld: cannot find -lcap
  collect2: error: ld returned 1 exit status

  Fix by adding gddbm and libcap as inputs. Yes, libcap is needed
  _in addition_ to libcap_ng (I tested removing libcap_ng, it failed).

Without this change, virt-viewer cannot be used with guests machines
that uses SPICE.