Fix KBUILD_MODNAME

[mmisono]

I found that bpftrace cannot include net/sock.h on Linux 5.7-rc7.

% sudo ./src/bpftrace -e 'BEGIN {}' --include net/sock.h
[...]
/lib/modules/5.7.0-rc7/source/include/net/flow_offload.h:342:3: error: array initializer must be an initializer list or string literal
/lib/modules/5.7.0-rc7/source/include/net/flow_offload.h:342:3: error: expected ';' at end of declaration

Related parts are

torvalds/linux@319a1d1#diff-c92be59c4d9bb3274ff03bce22e057b4R269
https://github.com/torvalds/linux/blob/444fc5cde64330661bf59944c43844e7d4c2ccd8/include/linux/netlink.h#L96

and it seems KBUILD_MODNAME is invalid. Use "bpftrace" instead of '"bpftrace"' as KBUILD_MODNAME.

Checklist

• Language changes are updated in docs/reference_guide.md
  • This is no language changes.
• User-visible and non-trivial changes updated in CHANGELOG.md

[mmisono]

The workaround without recompiling is

#define KBUILD_MODNAME "bpftrace"
#include <net/sock.h>

BEGIN {}