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 <[email protected]>

Author: cfriedt

subsys/testsuite/coverage/coverage.c

@@ -9,7 +9,7 @@
 #include <stdint.h>
 #include <errno.h>
 #include <string.h>
-#include <fnmatch.h>
+#include <zephyr/posix/fnmatch.h>
 #include "coverage.h"
 #include <zephyr/arch/common/semihost.h>
 #include <sys/types.h>