Third-party: Disable -Werror for flatcc build (pytorch#19049)

### Summary
Apple Clang 21 (Xcode 26) elevates -Wimplicit-int-conversion-on-negation
and -Wunterminated-string-initialization to errors, which breaks the
vendored flatcc build at pprintint.h and grisu3_print.h. The warnings
are benign and the fix belongs upstream in dvidelabs/flatcc; disable
-Werror locally via FLATCC_ALLOW_WERROR=OFF for both the flatcc_ep
external project (host compiler) and the in-tree flatccrt runtime lib.

### Test plan
```
./install_executorch.sh
```

Authored with Claude.

Author: rascani

third-party/CMakeLists.txt

@@ -142,6 +142,10 @@ set(FLATCC_DEBUG_CLANG_SANITIZE
     OFF
     CACHE BOOL ""
 )
+set(FLATCC_ALLOW_WERROR
+    OFF
+    CACHE BOOL ""
+)
 set(FLATCC_INSTALL
     OFF
     CACHE BOOL ""