protobuf-c: disable parallelism to avoid race condition

Unfortunately 489d3b4b93 did not completely
fix the problem - if you try cleaning and rebuilding protobuf-c-native it
doesn't take long to reproduce the issue on a 32-core machine. I spent
some time trying to debug this but failed, there is still a race between
generating t.test-full.pb.h and compiling cxx_generate_packed_data.c
despite BUILT_SOURCES and explicit dependencies. I even tried converting
the multiple target rules to use grouped targets (&:), that didn't fix it
either. Disabling parallelism as a workaround only costs ~20s and it
turns out that upstream is switching to Meson soon anyway:

  https://github.com/protobuf-c/protobuf-c/pull/340

Signed-off-by: Paul Eggleton <paul.eggleton@linux.microsoft.com>
Signed-off-by: Khem Raj <raj.khem@gmail.com>

meta-oe/recipes-devtools/protobuf/protobuf-c_1.3.3.bb

@@ -22,6 +22,11 @@ S = "${WORKDIR}/git"
 
 inherit autotools pkgconfig
 
+# After several fix attempts there is still a race between generating
+# t.test-full.pb.h and compiling cxx_generate_packed_data.c despite
+# BUILT_SOURCES and explicit dependencies.
+PARALLEL_MAKE = ""
+
 PACKAGE_BEFORE_PN = "${PN}-compiler"
 
 FILES_${PN}-compiler = "${bindir}"