[PGO][HIP][NFC] Fix hipModuleGetGlobal -Wunused-function warning (#207293)
The functions trigger the warning on Windows (without elf.h) and is
fatal under -Werror.
Fix by adding [[maybe_unused]]. Alternatively it could be moved inside
the existing __has_include(<elf.h>) block,; however that would trigger
-Wunused-but-set-global on pHipModuleGetGlobal.
Current fix is minimal and can be removed once hipModuleGetGlobal is
supported without elf.h.