Adjust compile flags for open62541 to build with gcc 9.

--- a/deps/open62541/CMakeLists.txt
+++ b/deps/open62541/CMakeLists.txt
@@ -407,6 +407,9 @@
                     -Wc++-compat
                     -fno-strict-aliasing # fewer compiler assumptions about pointer types
                     -fexceptions # recommended for multi-threaded C code, also in combination with C++ code
+                    -Wno-cast-function-type
+                    -Wno-restrict
+                    -Wno-sign-conversion
                     )
     set(CMAKE_C_FLAGS_DEBUG "${CMAKE_C_FLAGS_DEBUG} -Werror")