Add missing printf function attribute

Reported by clang:

    ../src/share/log.c:812:35: error: format string is not a string literal [-Werror,-Wformat-nonliteral]
      812 |                 r = vasprintf(&m, format, aq);
          |                                   ^~~~~~

Author: cgzones

src/share/log.h

@@ -124,7 +124,7 @@ int log_format_iovec(
                 bool newline_separator,
                 int error,
                 const char *format,
-                va_list ap);
+                va_list ap) _printf_(6,0);
 
 /* This modifies the buffer passed! */
 int log_dump_internal(