Add a pkgconfig file

[hobbitalastair]

The netsurf project uses a pkgconfig file to find linking flags for utf8proc. It would be nice to have this in the upstream library.

@tlsa this seems to be required to build against upstream instead of netsurf's fork.

[stevengj]

Seems reasonable to add.

[ianwlarson]

#108

[stevengj]

CLosed by #142.