fftools/cmdutils: constify the argument of get_rotation()

fftools/cmdutils.c

@@ -994,7 +994,7 @@ void *allocate_array_elem(void *ptr, size_t elem_size, int *nb_elems)
     return new_elem;
 }
 
-double get_rotation(int32_t *displaymatrix)
+double get_rotation(const int32_t *displaymatrix)
 {
     double theta = 0;
     if (displaymatrix)

fftools/cmdutils.h

@@ -461,6 +461,6 @@ void *allocate_array_elem(void *array, size_t elem_size, int *nb_elems);
     char name[16];\
     snprintf(name, sizeof(name), "%d", rate);
 
-double get_rotation(int32_t *displaymatrix);
+double get_rotation(const int32_t *displaymatrix);
 
 #endif /* FFTOOLS_CMDUTILS_H */