Fixes build errors for the third-party mongoose module:
```
In file included from
/nix/store/r5s3w32ahjzdlzsfrhybc3l2qcpi6yb2-libpcap-1.9.0/include/pcap.h:43,
from /build/ntopng-2.0/include/ntop_includes.h:93,
from src/HTTPserver.cpp:22:
/nix/store/r5s3w32ahjzdlzsfrhybc3l2qcpi6yb2-libpcap-1.9.0/include/pcap/pcap.h:958: note: this is the location of the previous definition
#define INVALID_SOCKET -1
src/../third-party/mongoose/mongoose.c:270:13: error: multiple types in one declaration
typedef int SOCKET;
^~~~~~
```
