protobuf-c: Export native protoc

configure detects protoc based on some pkgconfig logic which confuses
cross compilation and ends up choosing /usr/bin/protoc causing build
failures when host system has version mismatched protoc installed, it
works ok as long as compatible protoc is installed

Fixes
./protobuf-c/protobuf-c.pb.h:17:2: error: This file was generated by an older version of protoc which is
 ^

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

meta-oe/recipes-devtools/protobuf/protobuf-c_1.4.0.bb

@@ -19,6 +19,8 @@ S = "${WORKDIR}/git"
 
 inherit autotools pkgconfig
 
+export PROTOC = "${STAGING_BINDIR_NATIVE}/protoc"
+
 # 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.