linux-v4l2: Fix build with Clang 10.0

add_control_property() was previously static inline bool, but did not
return a value and failed to build on FreeBSD-CURRENT with Clang 10.0,
with:

error: non-void function 'add_control_property' should return a value

plugins/linux-v4l2/v4l2-controls.c

@@ -95,7 +95,7 @@ static inline bool valid_control(struct v4l2_queryctrl *qctrl)
 	return (qctrl->flags & INVALID_CONTROL_FLAGS) == 0;
 }
 
-static inline bool add_control_property(obs_properties_t *props,
+static inline void add_control_property(obs_properties_t *props,
 					obs_data_t *settings, int_fast32_t dev,
 					struct v4l2_queryctrl *qctrl)
 {