Fix warnings with Rust 1.46.0 - 1.51.0, inclusive

These ranges of rustc are pickier about where to place
an #[allow(unused_doc_comments)] attribute.  It caused warnings when
building for BSD-based targets.

src/macros.rs

@@ -113,9 +113,9 @@ macro_rules! libc_enum {
         $v enum $BitFlags {
             $($entries)*
         }
-        #[allow(unused_doc_comment)]
         impl ::std::convert::TryFrom<$repr> for $BitFlags {
             type Error = $crate::Error;
+            #[allow(unused_doc_comments)]
             fn try_from(x: $repr) -> $crate::Result<Self> {
                 match x {
                     $($try_froms)*