fix taglib include path, use what pkg-config provides

currently xbmc only compiles if taglib 1.8 includes are found in a default location (/usr/include/taglib /usr/local/include/taglib) although pkg-config is used to query the correct Libs and CFLAGS.

the taglib.pc file defines:
   Libs:  $taglib_install_prefix/lib 
   Cflags: $taglib_install_prefix/include/taglib

now using 
#include <taglib/somefile.h> translates to $taglib_install_prefix/include/taglib/taglib/somefile.h, which is wrong.

This PR allows building with taglib installed in arbitrary locations, as long as pkg-config finds it. (e.g. by setting PKG_CONFIG_PATH)

tested on linux