[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++.

gdb/po/gdbtext

diff --git a/gdb/po/gdbtext b/gdb/po/gdbtext
index 1f80cf9..9f30d37 100755
--- a/gdb/po/gdbtext
+++ b/gdb/po/gdbtext
@@ -31,7 +31,7 @@
     --files-from=- \
     --force-po \
     --debug \
-    --language=c \
+    --language=c++ \
     --keyword=_ \
     --keyword=N_ \
     ${__directories} \