Make error on restrict pointer: need C99 mode

[Matthew Vernacchia]

I got the following error when building:

error: expected ‘;’, ‘,’ or ‘)’ before ‘instance’
 void TRICAL_measurement_calibrate(TRICAL_instance_t *restrict instance,

I was able to fix this by including -std=c99 in the set(CMAKE_C_FLAGS string in CMakeLists.txt