lib: work around nonstandard bit-fields

lib: work around nonstandard bit-fields

Exported header files used by applications should allow the strictest
compiler flags. Language extensions used in many places must be explicitly
marked or removed to avoid warnings and compilation failures.

This commit prevents the following errors:

 error: type of bit-field `[...]' is a GCC extension

Note: the standard does not require implementations to issue a diagnostic
message with these, and such errors do not occur with recent GCC or clang
versions. However, GCC 4.7 is still common and using the extension keyword
is easier than checking compiler version.

Signed-off-by: Adrien Mazarguil <adrien.mazarguil@6wind.com>