Fix gcc11 arm build

What does this implement/fix?

when (cross-)compiling with arm-linux-gnueabihf-g++ (11.3.0) on ubuntu22.04 you get errors:

error: cannot convert ‘<brace-enclosed initializer list>’ to ‘Eigen::internal::Packet4f’ {aka ‘float32x4_t’} in return
[...]
error: cannot convert ‘Eigen::internal::Packet4f’ {aka ‘float32x4_t’} to ‘Eigen::internal::Packet2f’ {aka ‘float32x2_t’} in initialization

Additional information

SET(CMAKE_C_COMPILER /usr/bin/arm-linux-gnueabihf-gcc)
SET(CMAKE_CXX_COMPILER /usr/bin/arm-linux-gnueabihf-g++)

SET(CMAKE_C_FLAGS "-mcpu=cortex-a72 -mfloat-abi=hard -mfpu=neon-fp-armv8")
SET(CMAKE_CXX_FLAGS "${CMAKE_C_FLAGS}")