Fix compile error under Linux

[efferre79]

when building pytango-9.2.5 under Gentoo with gcc 7.3.0 I get the following error:

/var/tmp/portage/dev-python/pytango-9.2.5/work/pytango-9.2.5/ext/device_data.cpp:185:20: error: ‘False’ was not declared in this scope
             assert(False);
                    ^
/var/tmp/portage/dev-python/pytango-9.2.5/work/pytango-9.2.5/ext/device_data.cpp:185:20: note: suggested alternative: ‘raise’
error: command 'x86_64-pc-linux-gnu-g++' failed with exit status 1

[jairomoldes]

Seems OK. Actually "False" instead of "false" looks like a typo ("false" is used elsewhere in the code)

[efferre79]

changed the title because not specific to gcc version

[tiagocoutinho]

LGTM