package/xerces: needs dynamic library

package/xerces: needs dynamic library

Disable xerces on toolchains without dynamic library to avoid a static
build failure raised since the addition of xml-security-c in commit
bbda38afbbfa79ff436e1a187bb267b8d9558036 because xerces-c.pc does not
contain icu or iconv dependencies in Libs.Private

Fixes:
 - http://autobuild.buildroot.org/results/3e4f31eb017878d3417742cbc83e16bc848db98e

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>