basic/log: fix _printf_ annotation on log_object_internalv

Fixup for 4b58153dd2.

I saw this because of a clang warning. With gcc the -Wformat-nonliteral warning
doesn't seem to work as expected.

In two places, a string constructed with strjoina is used as the pattern. This
is safe, because we're taking a pattern which was already marked with _printf_
and prepending a known value to it.  Those places are marked with #pragma to
silence the warning.

src/basic/log.h

@@ -115,7 +115,7 @@ int log_object_internalv(
                 const char *extra_field,
                 const char *extra,
                 const char *format,
-                va_list ap) _printf_(9,0);
+                va_list ap) _printf_(10,0);
 
 int log_struct_internal(
                 int level,