Check whether headers are there at start of install script

[brandonheller]

Otherwise install.sh breaks partway through. Small issue.

[moijes12]

@brandonheller

By headers, do you mean the linux kernel headers ?

[brandonheller]

Probably; it's been two years since this bug was opened. I haven't looked at the code in awhile but I assume these are still needed to build mnexec.

[lantz]

We should check whether this is still a problem.

[cheriimoya]

i just checked this issue with a new ubuntu 20.04 vm, it works without installing anything!
simply boot up the vm (fresh install), download the repo as a zip archive from github, unzip and run the install script.

i think ubuntu comes with the headers preinstalled since 14.04 (https://askubuntu.com/questions/464659/are-linux-headers-installed-by-default-how-to-check-if-theyre-installed), so this shouldn't be an issue anymore

i also removed any linux headers from the vm and the install script still worked