Fix parsing of "private global" rules

[msm-code]

I've went ahead and tried to fix the issue I've found. I hope my C++ is adequate.

I tried to be careful not to introduce any regressions and memory leaks, but please do check.

I've also added a test to parser_tests.cpp but had problems building the test suite, so I just hope the test works. I tested my fixes by hand, using the python bindings instead:

import yaramod

test = """
/* comment test */
private global rule Kot
{
    strings:
        $a = "dummy1"

    condition:
        $a
}
/* comment test */
global private rule Hmm
{
    strings:
        $a = "dummy1"

    condition:
        $a
}

/* this will not work:
global global rule Zzz
{
    strings:
        $a = "dummy1"

    condition:
        $a
}
*/

private rule private_rule {
    condition:
        true
}

global rule global_rule {
    condition:
        true
}
"""

print(yaramod.Yaramod().parse_string(test).text_formatted)

for rule in yaramod.Yaramod().parse_string(test).rules:
    print(rule.name, ("private" if rule.is_private else ""), ("global" if rule.is_global else ""))

This test doesn't work on current master branch (as expected), but works after this PR.

Fixes #92.

[s3rvac]

Thank you for the very nicely written issue and PR! Since @metthal is on vacation, could I ask you @TadeasKucera to take a look at this PR and the associated issue #92?

[TadeasKucera]

First of all thank you very much for this PR! I have run the tests and all of them passed. I only have some minor remarks listed below.

[msm-code]

No problem, glad I can help!