libretro-common/samples/streams/rzip: silence fgets -Wunused-result

Minor bit-rot cleanup.  Every CI build of the libretro-common
samples workflow emitted:

  rzip.c:246:7: warning: ignoring return value of 'fgets' declared
    with attribute 'warn_unused_result' [-Wunused-result]

The sample prompts the user with "Overwrite? [Y/n]" and reads
the reply with fgets.  The existing code already handles fgets
failure correctly -- reply[0] is initialised to '\0' on line
242, so an EOF or read error leaves reply[0] == '\0' and the
subsequent "if (reply[0] != 'Y')" short-circuits to the safe
"don't overwrite" path.  The warning is the tool correctly
noticing that the return value is unused; the code is in fact
correct.

Fix: cast the fgets call to (void)! to explicitly signal that
the return is intentionally ignored.  Add a comment explaining
the fail-safe interaction with the reply[0] == '\0' init.  This
is the same (void)!<call> idiom used elsewhere in the repository
to silence GCC/glibc's warn_unused_result without changing
behaviour.

Verified:
  - CFLAGS += -Wall -Wunused-result -pedantic -std=gnu99:
    clean compile (warning gone, no new warnings).
  - Full libretro-common samples CI dry-run:
      Built: 15  Ran: 15  Failed: 0

  The only remaining warning in the full dry-run is the
  pre-existing "ISO C forbids an empty translation unit" from
  compat_snprintf.c, which is in the main library (not a
  sample) and has a different root cause -- separate concern.

Regression test: NONE.  This is a diagnostic cleanup on a
build-only sample; there is no behaviour change to discriminate.

Author: LibretroAdmin

libretro-common/samples/streams/rzip/rzip.c

@@ -243,7 +243,13 @@ int main(int argc, char *argv[])
 
       printf("WARNING: Output file already exists: %s\n", out_file_path);
       printf("         Overwrite? [Y/n]: ");
-      fgets(reply, sizeof(reply), stdin);
+      /* Ignore the fgets return value intentionally -- on EOF or
+       * error we fall through to the reply[0] check below, which
+       * fails the 'Y' comparison (reply[0] stays '\0' from the
+       * initialisation above) and bails out safely without
+       * overwriting.  Cast to void to silence -Wunused-result on
+       * GCC/glibc, which attaches warn_unused_result to fgets. */
+      (void)!fgets(reply, sizeof(reply), stdin);
       if (reply[0] != 'Y')
          goto end;
    }