libxml++: Disable parallel make in ptest compile

This fails to build in parallel, perhaps needs to fix the component
makery

Fixes
| chmod +x dom_build/make_check.sh
| echo '# Generated and used by "make check"' >dom_parse_entities/make_check.sh
| /bin/bash: dom_parse_entities/make_check.sh: No such file or directory
| make[1]: *** [Makefile:1807: dom_parse_entities/make_check.sh] Error 1
| make[1]: Leaving directory '/home/pokybuild/yocto-worker/meta-oe/build/build/tmp/work/core2-64-poky-linux/libxml++/2.42.1-r0/build/examples'
| make: *** [Makefile:1812: buildtest] Error 2

Signed-off-by: Khem Raj <raj.khem@gmail.com>

meta-oe/recipes-core/libxml/libxml++_2.42.1.bb

@@ -27,6 +27,8 @@ do_compile_ptest() {
   oe_runmake -C examples buildtest
 }
 
+PTEST_PARALLEL_MAKE = ""
+
 FILES:${PN}-doc += "${datadir}/devhelp"
 FILES:${PN}-dev += "${libdir}/libxml++-2.6/include/libxml++config.h"