elfutils/flex: Disable parallel make ptest compile

The recent change enabling parallel make for ptest compile/install caused
autobuilder failures in these two recipes. Disable parallel make here
for now until someone can debug the race and get it fixed (preferably
upstream).

(From OE-Core rev: 12755e3e771eb2f1628e2b3dd7138c8766973d82)

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta/recipes-devtools/elfutils/elfutils_0.187.bb

@@ -56,6 +56,7 @@ do_compile_ptest() {
 	cd ${B}/tests
 	oe_runmake buildtest-TESTS oecheck
 }
+PTEST_PARALLEL_MAKE = ""
 
 do_install_ptest() {
 	if [ ${PTEST_ENABLED} = "1" ]; then