BUG: fix eplicit_padding when a struct ends in a bitfield.

A struct ending in a bitfield will have its trailing padding
duplicated. This change skips adding the trailing padding if the last
field was a bitfield, and thus, already has already been padded.

Author: sw17ch

bindgen-tests/tests/expectations/tests/explicit-padding.rs

@@ -15,3 +15,8 @@ union dont_pad_me {
         uint32_t second;
         uint16_t third;
 };
+
+struct also_pad_me {
+        uint16_t first;
+        uint8_t bits: 1;
+};

bindgen-tests/tests/headers/explicit-padding.h

@@ -285,6 +285,11 @@ impl<'a> StructLayoutTracker<'a> {
             return None;
         }
 
+        // If the last field was a bitfield, it will already have been padded.
+        if self.last_field_was_bitfield {
+            return None;
+        }
+
         // Padding doesn't make sense for rust unions.
         if self.is_rust_union {
             return None;