header_enum_explicit_type_hpp test fails on aarch64

[nivkner]

The error is:

rust-bindgen/tests/headers/enum_explicit_type.hpp:9:16: error: enumerator value evaluates to -1, which cannot be narrowed to type 'char' [-Wc++11-narrowing], err: true

I might be wrong but I think the problem is that char in c++ (when neither signed nor unsigned) is implementation defined.
In x86 char is defined as signed and in aarch64 it is unsigned, as can be seen by the fact that in the libc crate, c_char is an alias to i8 and u8 in each platform respectively.

The flag are default when running cargo test

Device: odroid C2

Linux kernel: 3.14.79-27-ARCH

Clang: 4.0.1

GCC: 7.1.1 20170516

Host: nightly-aarch64-unknown-linux-gnu

Rustc: 1.21.0-nightly (aac223f4f 2017-07-30)