llext: fix llext_find_sym() not to return a "const" value

Returned values are copies, so trying to "const" return values cannot
have any effect.

Fixes the following compiler warning:
```
llext.h:165: warning: type qualifiers ignored on function return type
```

Fixes commit 41e0a4a3 ("llext: Linkable loadable extensions")

Signed-off-by: Marc Herbert <marc.herbert@intel.com>