[HIP] Fix compiler error and warnings

[noelchalmers]

Description

Fixes #546 and addresses a couple other compiler warnings when compiling with HIP support.

[codecov]

Codecov Report

Merging #547 (52b3347) into main (7dd3c62) will decrease coverage by 0.00%.
The diff coverage is n/a.

❗ Current head 52b3347 differs from pull request most recent head 631142a. Consider uploading reports for the commit 631142a to get more accurate results

@@            Coverage Diff             @@
##             main     #547      +/-   ##
==========================================
- Coverage   76.35%   76.34%   -0.01%     
==========================================
  Files         262      262              
  Lines       19522    19520       -2     
==========================================
- Hits        14906    14903       -3     
- Misses       4616     4617       +1     

Impacted Files	Coverage Δ
src/occa/internal/lang/specialMacros.cpp	59.60% <0.00%> (-0.67%)	⬇️
src/occa/internal/utils/sys.cpp	76.45% <0.00%> (-0.11%)	⬇️

[kris-rowe]

Looks good.