Squelch -Wformat-nonliteral in pcap_vasprintf().

Squelch -Wformat-nonliteral in pcap_vasprintf().

On FreeBSD 13.1/AArch64 with Clang 13.0.0 "make asprintf.o" produced two
warnings:

./missing/asprintf.c:64:36: warning: format string is not a string
  literal [-Wformat-nonliteral]
        len = vsnprintf(&buf, sizeof buf, format, args);
                                          ^~~~~~
./missing/asprintf.c:75:33: warning: format string is not a string
  literal [-Wformat-nonliteral]
        ret = vsnprintf(str, str_size, format, args);
                                       ^~~~~~

Add printf()-related attributes to the prototype, as is already done for
pcap_asprintf().