Use c99 static array size declarations in exported functions too
It seems quite useful to provide this additional information in public exported
functions.
This is a c99 feature, not supported in C++. Without the check in _sd-common.h:
FAILED: test-bus-vtable-cc@exe/src_libsystemd_sd-bus_test-bus-vtable-cc.cc.o
...
In file included from ../src/libsystemd/sd-bus/test-bus-vtable-cc.cc:9:
In file included from ../src/systemd/sd-bus-vtable.h:26:
In file included from ../src/systemd/sd-bus.h:26:
../src/systemd/sd-id128.h:38:47: error: static array size is a C99 feature, not permitted in C++
char *sd_id128_to_string(sd_id128_t id, char s[static SD_ID128_STRING_MAX]);
^
In .c files, I opted to use the define for consistency, even though we don't support
compilation with a C++ compiler, so the unconditional keyword would work too.