dbus-protocol.h: compile under C++11

dbus-protocol.h: compile under C++11

C++11 compilers have a feature called 'user-defined string literals' which
allow arbitrary string suffixes to have user-defined meaning.

This makes code that concatenates macros with string literals without
intervening whitespace illegal under C++11. Fortunately, string literal
concatenation has allowed intervening whitespace since the dawn of time,
so the solution is to simply pad with spaces.

Tested (header) with GCC 4.7 (trunk).

Bug: https://bugs.freedesktop.org/show_bug.cgi?id=46147
Reviewed-by: Simon McVittie <simon.mcvittie@collabora.co.uk>