swf: Don't error if both libflate and flate2 features are enabled

Fixes errors I was getting from rust-analyzer checking with all
features enabled.

swf/src/read.rs

@@ -133,7 +133,7 @@ fn make_zlib_reader<'a, R: Read + 'a>(input: R) -> Result<Box<dyn Read + 'a>> {
     Ok(Box::new(ZlibDecoder::new(input)))
 }
 
-#[cfg(feature = "libflate")]
+#[cfg(all(feature = "libflate", not(feature = "flate2")))]
 fn make_zlib_reader<'a, R: Read + 'a>(input: R) -> Result<Box<dyn Read + 'a>> {
     use libflate::zlib::Decoder;
     let decoder = Decoder::new(input)?;

swf/src/write.rs

@@ -88,7 +88,7 @@ fn write_zlib_swf<W: Write>(mut output: W, swf_body: &[u8]) -> Result<()> {
     Ok(())
 }
 
-#[cfg(feature = "libflate")]
+#[cfg(all(feature = "libflate", not(feature = "flate2")))]
 fn write_zlib_swf<W: Write>(mut output: W, swf_body: &[u8]) -> Result<()> {
     use libflate::zlib::Encoder;
     let mut encoder = Encoder::new(&mut output)?;