configure.ac: remove explicit default path for libxml2

OpenEmbedded needs to cross-compile rrdtool, and historically has
applied this patch locally because the implementation of EX_CHECK_ALL
unconditionally added -I/usr/include.  Since the correct include path is
automatically provided by a successful pkg-config invocation there is no
need to set it explicitly.

Signed-off-by: Peter A. Bigot <pab@pabigot.com>
Merge pull request #524 from pabigot/support-cross-compile

configure.ac: remove explicit default path for libxml2