Mark GenericByteArray::new_unchecked unsafe (#4584)

diff --git a/arrow-array/src/array/byte_array.rs b/arrow-array/src/array/byte_array.rs
index 0a18062..be10a45 100644
--- a/arrow-array/src/array/byte_array.rs
+++ b/arrow-array/src/array/byte_array.rs
@@ -159,7 +159,7 @@
     /// # Safety
     ///
     /// Safe if [`Self::try_new`] would not error
-    pub fn new_unchecked(
+    pub unsafe fn new_unchecked(
         offsets: OffsetBuffer<T::Offset>,
         values: Buffer,
         nulls: Option<NullBuffer>,