libpcap needs to be declared as dependency

After a successful ./configure:

```
# make
g++ -c -Wall -g -O2 -std=c++11 -DFUZZER_VERSION=0.1 -Iinclude main.cpp -o main.o
In file included from /usr/include/tins/tins.h:49:0,
                 from main.cpp:25:
/usr/include/tins/packet_writer.h:35:18: fatal error: pcap.h: No such file or directory
 #include <pcap.h>
                  ^
compilation terminated.
make: *** [main.o] Error 1
```
