Consider alignment contribution from bitfields

[nbdd0121]

Partial fix to https://lore.kernel.org/rust-for-linux/Y8Ax%[email protected]/ (not a full fix, because it short below is replaced with a int then the code still fails to compile)

Currently binding for code like this wouldn't compile:

struct outer {
    struct {
        short a: 16;
        short b: 16;
    };
} __attribute__((packed));

because bindgen sticks an extra #[repr(align(2))] on the inner struct.

This extra alignment is not necessary and can be removed. The removed NB line in this PR is no longer relevant because of the addition _bitfield_align field generated starting in #1950.

cc @emilio @ojeda