a/math: Add stdbool include to m_lowpass_integer.h

In the file m_lowpass_integer.h in the dir src/xrt/auxiliary/math/m_lowpass_integer.h there is a function on line 74 that is

bool
m_lowpass_integer_is_initialized(const struct m_lowpass_integer *mli);

however this header file does not #include<stdbool.h> the only thing it includes is #include<stdint.h>.

however this compiles fine on Monado.

however it does not compile fine when I am doing my shenanigans with the header files.

This does not currently break anything in monado itself, because the file is not being used, but it does break some things on my end.