libxml2: don't override XML_CATALOG_FILES in xmllint wrapper if already set

libxml2: don't override XML_CATALOG_FILES in xmllint wrapper if already set

The KDE build uses custom catalogs by setting XML_CATALOG_FILES, so this
wrapper should not override that value if it has already been set.

[RP: Add vardepsexclude since bitbake stores the expanded version of the variable
name in the siginfo data which would expand to a full build path in the native
case]

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>