Found the solution on github
add:
SET_PERI_REG_MASK(SENS_SAR_PERI_CLK_GATE_CONF_REG, SENS_IOMUX_CLK_EN_M);

(you'll also need #include "soc/sens_reg.h" at the top of the file)

I am facing the same problem.
Did you find the solution on how to read a GPIO in the stub?
thanks