package/clamav: fix libxml2 build

Pass ac_cv_path_xmlconfig to fix the following build failure with
libxml2 raised since bump to version 0.102.0 in commit
007f4996 and
https://github.com/Cisco-Talos/clamav/commit/083ca38727b0ebd3979cf1ebc7b31148f6d5e7f7:

aarch64-none-linux-gnu-gcc: WARNING: unsafe header/library path used in cross-compilation: '-I/usr/include/libxml2'
In file included from /usr/include/libxml2/libxml/parser.h:810,
                 from others.c:62:
/usr/include/libxml2/libxml/encoding.h:31:10: fatal error: unicode/ucnv.h: No such file or directory
   31 | #include <unicode/ucnv.h>
      |          ^~~~~~~~~~~~~~~~

Fixes:
 - http://autobuild.buildroot.org/results/5bb3bea6d13ac8d497f6862374d65a59bea7ed09



Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>