[gdb/i18n] Use c++ in po/gdbtext

[gdb/i18n] Use c++ in po/gdbtext

I came across a script gdb/po/gdbtext:
...
$ ./po/gdbtext
Usage: ./po/gdbtext <xgettext> <package>  <directory> ...
...

When using it, I get:
...
$ ./po/gdbtext xgettext gdb ../gdb ../gdbsupport/ ../gdbserver/
unittests/memory-map-selftests.c:30: warning: unterminated string literal
unittests/memory-map-selftests.c:41: warning: unterminated string literal
$
...
due to raw strings being used in unittests/memory-map-selftests.c.

Fix this by updating the language used in ./po/gdbtext to C++.