core: fix includes in utils.c causing compilation failure because of undefined be64toh/htobe64

[sedmicha]

#include <ipfixcol2.h> resulted in including a file in which be64toh/htobe64 functions were used. This would clash with the defines present here that are necessary to get the wanted strerror_r function, resulting in the be64toh/htobe64 functions being undefined.

This was fixed by not including ipfixcol2.h as a whole, but only the necessary part, as well as adding explicit includes for the system headers that were brought by ipfixcol2.h.