utils - fix static_assert function to c++11 standard

[codecov-commenter]

Codecov Report

Patch coverage has no change and project coverage change: -0.03 :warning:

Comparison is base (1f4b212) 40.85% compared to head (0d90602) 40.82%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #169 +/- ## ========================================== - Coverage 40.85% 40.82% -0.03% ========================================== Files 88 88 Lines 8499 8499 Branches 1285 1285 ========================================== - Hits 3472 3470 -2 - Misses 4264 4266 +2 Partials 763 763 ``` | Flag | Coverage Δ | | |---|---|---| | tests | `40.82% <ø> (-0.03%)` | :arrow_down: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=CESNET#carryforward-flags-in-the-pull-request-comment) to find out more. | [Impacted Files](https://app.codecov.io/gh/CESNET/ipfixprobe/pull/169?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=CESNET) | Coverage Δ | | |---|---|---| | [utils.cpp](https://app.codecov.io/gh/CESNET/ipfixprobe/pull/169?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=CESNET#diff-dXRpbHMuY3Bw) | `64.44% <ø> (ø)` | | ... and [1 file with indirect coverage changes](https://app.codecov.io/gh/CESNET/ipfixprobe/pull/169/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=CESNET)

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