memdebug.h: avoid `-Wredundant-decls` with an extra guard

memdebug.h: avoid `-Wredundant-decls` with an extra guard

Add an extra guard for the function and variable declarations to avoid
redundant redeclaration warnings when including this header multiple
times. This can happen in unity builds when including it again after
`curl_memory.h`.

Fixes:
```
bld/tests/server/CMakeFiles/servers.dir/Unity/unity_0_c.c
In file included from lib/mprintf.c:32,
                 from bld/tests/server/CMakeFiles/servers.dir/Unity/unity_0_c.c:7:
lib/memdebug.h:52:14: error: redundant redeclaration of ‘curl_dbg_logfile’ [-Werror=redundant-decls]
   52 | extern FILE *curl_dbg_logfile;
      |              ^~~~~~~~~~~~~~~~
In file included from tests/server/resolve.c:50,
                 from bld/tests/server/server_bundle.c:7,
                 from bld/tests/server/CMakeFiles/servers.dir/Unity/unity_0_c.c:4:
lib/memdebug.h:52:14: note: previous declaration of ‘curl_dbg_logfile’ with type ‘FILE *’
   52 | extern FILE *curl_dbg_logfile;
      |              ^~~~~~~~~~~~~~~~
[...]
lib/memdebug.h:110:17: error: redundant redeclaration of ‘curl_dbg_fclose’ [-Werror=redundant-decls]
  110 | CURL_EXTERN int curl_dbg_fclose(FILE *file, int line, const char *source);
      |                 ^~~~~~~~~~~~~~~
lib/memdebug.h:110:17: note: previous declaration of ‘curl_dbg_fclose’ with type ‘int(FILE *, int,  const char *)’
  110 | CURL_EXTERN int curl_dbg_fclose(FILE *file, int line, const char *source);
      |                 ^~~~~~~~~~~~~~~
```
Ref: https://github.com/curl/curl/actions/runs/13822010778/job/38669360980#step:39:55

Cherry-picked from #15000
Closes #16696