Force inlining of the inline functions

[xen0n]

inline is not a requirement but only a suggestion, hence the compiler
has the freedom to not really compile the marked functions as inline.
Under some situations the affected functions could end up undefined in
the compiled library, causing its loading to fail!

For GCC at least, adding an always_inline attribute is enough to force
inlining. Not sure if this breaks other compilers but I cannot test, so
the definition is guarded by a check for GCC.

Really fixes #180, which is also a problem on, at least, Gentoo with GCC
5.3.0.

Evidence:

(before)
$ nm ujson.gcc-4.9.3.so | grep 'strreverse\|Buffer_AppendShortHex'
0000000000007557 T Buffer_AppendShortHexUnchecked
0000000000007557 t Buffer_AppendShortHexUnchecked.localalias.1
000000000000805d T strreverse
000000000000805d t strreverse.localalias.0

$ nm ujson.gcc-5.3.0.so | grep 'strreverse\|Buffer_AppendShortHex'
                 U Buffer_AppendShortHexUnchecked
                 U strreverse

(after)
$ nm ujson.gcc-5.3.0-always_inline.so | grep 'strreverse\|Buffer_AppendShortHex'
(no output)

[Jahaja]

I haven't been able to repro this when trying on FreeBSD but let us know if this still is an issue after #238.

[sdlarsen]

I still get this error.

import ujson ImportError: /home/.../venv/np_service/lib/python3.5/site-packages/ujson.cpython-35m-x86_64-linux-gnu.so: undefined symbol: Buffer_AppendShortHexUnchecked
I've tried with --no-binary all as well to no avail

My gcc: gcc (Gentoo 5.4.0 p1.0, pie-0.6.5) 5.4.0