compilation issues on debian

[alan-maguire]

as reported by https://github.com/MmartinezV2 in #11 (comment)

Two problems for build:

On file include/bpftune/bpftune.bpf.h fails because NULL is undefined.
Solved by adding at the begining of the file:
#ifndef NULL
#define NULL (0)
#endif
File bpf/libbpf_version.h does not exist
Solved by commenting line 54 #include <bpf/libbpf_version.h> in file src/libbpftune.c