cmake: Add -I${PROTOBUF_INCLUDE_DIR} to protoc invocation

Apparently in some cases the protobuf compiler is unable to find its own
files, and this fix has been reported to work.

See https://github.com/protobuf-c/protobuf-c/issues/491.

build-cmake/CMakeLists.txt

@@ -101,7 +101,7 @@ SET(CMAKE_CXX_STANDARD_REQUIRED ON)
 SET(CMAKE_CXX_EXTENSIONS OFF)
 ADD_CUSTOM_COMMAND(OUTPUT protobuf-c/protobuf-c.pb.cc protobuf-c/protobuf-c.pb.h
                    COMMAND ${PROTOBUF_PROTOC_EXECUTABLE}
-                   ARGS --cpp_out ${CMAKE_CURRENT_BINARY_DIR} -I${MAIN_DIR} ${MAIN_DIR}/protobuf-c/protobuf-c.proto)
+                   ARGS --cpp_out ${CMAKE_CURRENT_BINARY_DIR} -I${PROTOBUF_INCLUDE_DIR} -I${MAIN_DIR} ${MAIN_DIR}/protobuf-c/protobuf-c.proto)
 FILE(GLOB PROTOC_GEN_C_SRC ${MAIN_DIR}/protoc-c/*.h ${MAIN_DIR}/protoc-c/*.cc )
 ADD_EXECUTABLE(protoc-gen-c ${PROTOC_GEN_C_SRC} protobuf-c/protobuf-c.pb.cc protobuf-c/protobuf-c.pb.h)
 target_include_directories(protoc-gen-c