libintl.h: Include into :: namespace rather than C:: namespace.
This avoids a conflict with recent 20230802.1 where libintl.h functions get included into :: before we can include it in telephony-service code.
This namespace mismatch only shows up with libtelephonyservice/ contactwatcher.cpp, indicator/messagingmenu.cpp, indicator/ authhandler.cpp and maybe others. To make the include statements uniform across all of the telephony-service code, the C:: namespace inclusion of libintl.h functions has been dropped in all code files of telephony-service.
See: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=1068655