Fix VmathTimer

Issue/feature addressed

Variables time and count are not declared before use. This causes a compilation error when the compilation flag NEKTAR_BUILD_UNIT_TEST is used along with SIMD and LIKWID.

See example LIKWID: Marker API in a C/C++ application (rrze-hpc.github.io)

Proposed solution

Implementation

Tests

Suggested reviewers

@CFD-Xing

Notes

Please add any other information that could be useful for reviewers.

Checklist

• [ ] Functions and classes, or changes to them, are documented.
• [ ] User guide/documentation is updated.
• Changelog is updated.
• [ ] Suitable tests added for new functionality.
• Contributed code is correctly formatted. (See the contributing guidelines).
• License added to any new files.
• No extraneous files have been added (e.g. compiler output or test data files).