Include stdint.h to fix the uint8_t and uint16_t type missing error

[allenscmo]

Fix the issue: #50

During unitree_mujoco installation:

cd unitree_mujoco/simulate
mkdir build && cd build
cmake ..
make -j4

The make command failed with the following errors:

/home/unitree/unitree_mujoco/simulate/src/joystick/jstest.cc:30:5: error: ‘uint8_t’ does not name a type
   30 |     uint8_t left : 1;
      |     ^~~~~~~
/home/unitree/unitree_mujoco/simulate/src/joystick/jstest.cc:30:5: note: ‘uint8_t’ is defined in header ‘<cstdint>’; did you forget to ‘#include <cstdint>’?
/home/unitree/unitree_mujoco/simulate/src/joystick/jstest.cc:32:3: error: ‘uint16_t’ does not name a type
   32 |   uint16_t value;
      |   ^~~~~~~~

This PR update simulate/src/joystick/jstest.cc to add the missing header include.

Please review, thank you.

[allenscmo]

@DonovanZhu @wuyangning @Bin-Ro @robotlearning123 Would you please review this PR when you are free, this's a minor but blocker issue.