bugfix: firemon: skip coredump if unsupported

[kmk3]

The coredump-related code fails to build on Linux kernel version 3.8 as
apparently it only exists on Linux since version 3.10:

docker run --platform linux/386 --rm -it satmandu/crewbuild:386
[...]
./configure && make
[...]
gcc -ggdb -O2 -DVERSION='"0.9.73"' [...] -march=i686  -c ../../src/firemon/procevent.c -o ../../src/firemon/procevent.o
../../src/firemon/procevent.c: In function ‘procevent_monitor’:
../../src/firemon/procevent.c:399:38: error: ‘PROC_EVENT_COREDUMP’ undeclared (first use in this function); did you mean ‘PROC_EVENT_COMM’?
  399 |                                 case PROC_EVENT_COREDUMP:
      |                                      ^~~~~~~~~~~~~~~~~~~
      |                                      PROC_EVENT_COMM
../../src/firemon/procevent.c:399:38: note: each undeclared identifier is reported only once for each function it appears in
../../src/firemon/procevent.c:400:66: error: ‘union <anonymous>’ has no member named ‘coredump’
  400 |                                         pid = proc_ev->event_data.coredump.process_tgid;
      |                                                                  ^
make[1]: *** [../../src/prog.mk:25: ../../src/firemon/procevent.o] Error 1
make[1]: Leaving directory '/home/chronos/user/firejail/src/firemon'
make: *** [Makefile:72: src/firemon/firemon] Error 2

Environment: gcc 14.1.0, glibc 2.23 and linuxheaders 3.8 on ChromeOS
M58.

Misc: @Zopolis4 also reports that "All i686 chromebooks have a kernel
version of 3.8".

This amends commit e11949a ("add support for comm, coredump, and prctl
procevents in firemon", 2024-04-30).

Fixes #6414.

Reported-by: @Zopolis4

[kmk3]

@Zopolis4

Does this work?

Also, what is the version of the Linux API headers?

On Arch:

$ pacman -Q linux-api-headers
linux-api-headers 6.8-1

[glitsj16]

LGTM

[Zopolis4]

Does this work?

Yes.

Also, what is the version of the Linux API headers?

Linux header version:

$ crew search -v linuxheaders
linuxheaders: Linux headers for Chrome OS.
https://kernel.org/
Version: 3.8
License: GPL-2