Silence "may be used uninitialized" warnings

Possibly can be fixed upstream:

```
INFO: From Compiling third_party/zstd-jni/src/main/native/jni_zdict.c:
third_party/zstd-jni/src/main/native/jni_zdict.c: In function 'Java_com_github_luben_zstd_Zstd_trainFromBuffer':
third_party/zstd-jni/src/main/native/jni_zdict.c:55:12: warning: 'size' may be used uninitialized in this function [-Wmaybe-uninitialized]
 E1: return size;
            ^
third_party/zstd-jni/src/main/native/jni_zdict.c: In function 'Java_com_github_luben_zstd_Zstd_trainFromBufferDirect':
third_party/zstd-jni/src/main/native/jni_zdict.c:89:12: warning: 'size' may be used uninitialized in this function [-Wmaybe-uninitialized]
 E1: return size;
            ^
```

third_party/zstd-jni/BUILD.bazel

@@ -61,6 +61,7 @@ cc_binary(
         "-Wstrict-prototypes",
         "-Wno-unused-variable",
         "-Wpointer-arith",
+        "-Wno-maybe-uninitialized",
     ],
     includes = [
         "src/main/native",