Correctly handle linkage qualifiers for dpcpp device functions.

[kris-rowe]

Description

• SYCL device functions can only be declared with SYCL_EXTERNAL if they have external linkage under the normal C++ rules.
• Avoids adding the SYCL_EXTERNAL macro to functions declared with the static qualifier.

Closes #660

[codecov[bot]]

Codecov Report

Merging #662 (0390409) into development (662cd0a) will not change coverage. The diff coverage is 100.00%.

:exclamation: Current head 0390409 differs from pull request most recent head b5448b9. Consider uploading reports for the commit b5448b9 to get more accurate results

Additional details and impacted files

[](https://codecov.io/gh/libocca/occa/pull/662?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=libocca) ```diff @@ Coverage Diff @@ ## development #662 +/- ## ============================================ Coverage 76.30% 76.30% ============================================ Files 291 291 Lines 18778 18778 ============================================ Hits 14328 14328 Misses 4450 4450 ``` | [Impacted Files](https://codecov.io/gh/libocca/occa/pull/662?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=libocca) | Coverage Δ | | |---|---|---| | [src/occa/internal/lang/modes/dpcpp.cpp](https://codecov.io/gh/libocca/occa/pull/662?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=libocca#diff-c3JjL29jY2EvaW50ZXJuYWwvbGFuZy9tb2Rlcy9kcGNwcC5jcHA=) | `82.78% <100.00%> (+0.11%)` | :arrow_up: | ... and [2 files with indirect coverage changes](https://codecov.io/gh/libocca/occa/pull/662/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=libocca)