package/gnuradio: fix qtgui build when gr-analog is not set

package/gnuradio: fix qtgui build when gr-analog is not set

gr-qtgui examples needs to have gr-analog enabled, without this dependency
compile crash with:

In file included from
/x/output/build/gnuradio-3.8.1.0/gr-qtgui/examples/c++/display_qt.cc:22:
/x/output/build/gnuradio-3.8.1.0/gr-qtgui/examples/c++/display_qt.h:24:10:
fatal error: gnuradio/analog/noise_source.h: No such file or directory
24 | #include <gnuradio/analog/noise_source.h>
| ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
compilation terminated.
make[3]: *** [gr-qtgui/examples/c++/CMakeFiles/display_qt.dir/build.make:67:
gr-qtgui/examples/c++/CMakeFiles/display_qt.dir/display_qt.cc.o] Error 1
make[3]: *** Waiting for unfinished jobs....
In file included from
/somewhere/gnuradio/build/gr-qtgui/examples/c++/moc_display_qt.cpp:10:
/somewhere/gnuradio/build/gr-qtgui/examples/c++/../../../../gr-qtgui/examples/c++/display_qt.h:24:10:
fatal error: gnuradio/analog/noise_source.h: No such file or directory
24 | #include <gnuradio/analog/noise_source.h>
| ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
compilation terminated.

GR_ANALOG is not an explicit dependency of GR_QTGUI, so disable c++ examples if
user has not selected this option.

[backported from 7470a7a3771dd90defb826b464dfe62977cb1eb6]

Fixes:
- http://autobuild.buildroot.net/results/fde670499289f3d7d47379eebccf6e0f92c6d200/

Signed-off-by: Gwenhael Goavec-Merou <gwenhael.goavec-merou@trabucayre.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>