testsuite: coverage: use correct path to fnmatch.h
The minimal libc does not include `fnmatch.h` in its default search path.
This causes an issue when running coverage in CI as shown here.
https://github.com/zephyrproject-rtos/zephyr/actions/runs/18839500380/\
job/53748166725
That would technically be fixed with #97855 as well.
Testing Done:
```
west twister -c -p qemu_x86/atom -s kernel.pending.minimallibc --coverage\
--coverage-tool gcovr
```
Signed-off-by: Chris Friedt <cfriedt@tenstorrent.com>
1 file changed
