test/test_hs_descriptor: Don't invoke GCC warning when compiling with clang

Compiling with clang-15 yields the following errors:

```
  src/test/test_hs_descriptor.c:36:1: error: pragma diagnostic pop could not pop, no matching push [-Werror,-Wunknown-pragmas]
  ENABLE_GCC_WARNING("-Woverlength-strings")
  ^
  ./src/lib/cc/compat_compiler.h:104:11: note: expanded from macro 'ENABLE_GCC_WARNING'
            PRAGMA_DIAGNOSTIC_(pop)
            ^
  ./src/lib/cc/compat_compiler.h:94:35: note: expanded from macro 'PRAGMA_DIAGNOSTIC_'
  #    define PRAGMA_DIAGNOSTIC_(x) PRAGMA_(clang diagnostic x)
                                    ^
  ./src/lib/cc/compat_compiler.h:92:22: note: expanded from macro 'PRAGMA_'
  #  define PRAGMA_(x) _Pragma (#x)
                       ^
  <scratch space>:188:19: note: expanded from here
   clang diagnostic pop
                    ^
  1 error generated.
```

Do not invoke `ENABLE_GCC_WARNING("-Woverlength-strings")` when we are
compiling with clang. This issue was introduced by commit 3ce2304c
("Use new ENABLE/DISABLE_GCC_WARNING").

Cc: Nick Mathewson <nickm@torproject.org>
Signed-off-by: Ammar Faizi <ammarfaizi2@gnuweeb.org>
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