[COMPILE] fails + fixes

[Ewael]

Hi,

I've cloned the repo today (22.11.2020) and it does not compile.

 /opt/pkt2flow git:(master) scons
scons: Reading SConscript files ...
scons: done reading SConscript files.
scons: Building targets ...
gcc -o flow_db.o -c -I. flow_db.c
gcc -o pkt2flow.o -c -I. pkt2flow.c
pkt2flow.c: In function 'resemble_file_path':
pkt2flow.c:150:8: warning: implicit declaration of function 'asprintf'; did you mean 'vsprintf'? [-Wimplicit-function-declaration]
  150 |  ret = asprintf(&outputpath, "%s/%s", outputdir, type_folder);
      |        ^~~~~~~~
      |        vsprintf
gcc -o utilities.o -c -I. utilities.c
utilities.c: In function 'new_file_name':
utilities.c:62:9: warning: implicit declaration of function 'asprintf'; did you mean 'vsprintf'? [-Wimplicit-function-declaration]
   62 |   ret = asprintf(&fname, "%s_%"PRIu16"_%s_%"PRIu16"_%lu.pcap",
      |         ^~~~~~~~
      |         vsprintf
gcc -o pkt2flow flow_db.o pkt2flow.o utilities.o -L/usr/local/lib -L/usr/lib -lpcap
/bin/ld: pkt2flow.o:(.bss+0x0): multiple definition of `pairs'; flow_db.o:(.bss+0x0): first defined here
collect2: error: ld returned 1 exit status
scons: *** [pkt2flow] Error 1
scons: building terminated because of errors.

I've fixed both errors by adding

#define _GNU_SOURCE

in both pkt2flow.c and utilities.c.
I've also removed the pairs struct definition in flow_db.c because it already includes the one defined in pkt2flow.h.

I will make a pull request with the fixes, you should check it out.

Thank's!

EDIT: I can not PR the fixes because I do not have the permissions to do so, you might also check this out.