buffer impl: add cast for android compilation (#12583)

Commit Message: add cast for android compilation
Risk Level: low using the expected type for the constructor as the static_cast type.
Testing: local build of envoy mobile for android. CI

Signed-off-by: Jose Nino <jnino@lyft.com>

source/common/buffer/buffer_impl.h

@@ -40,7 +40,7 @@ class Slice : public SliceData {
   // SliceData
   absl::Span<uint8_t> getMutableData() override {
     RELEASE_ASSERT(isMutable(), "Not allowed to call getMutableData if slice is immutable");
-    return {base_ + data_, reservable_ - data_};
+    return {base_ + data_, static_cast<absl::Span<uint8_t>::size_type>(reservable_ - data_)};
   }
 
   /**