overdue refactor: compile all watch modules separately

watch-library/watch/watch_gpio.h

@@ -21,8 +21,12 @@
  * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
  * SOFTWARE.
  */
+#ifndef _WATCH_GPIO_H_INCLUDED
+#define _WATCH_GPIO_H_INCLUDED
 ////< @file watch_gpio.h
 
+#include "watch.h"
+
 /** @addtogroup gpio Digital Input and Output
   * @brief This section covers functions related to general-purpose input and output signals.
   */
@@ -69,3 +73,4 @@ void watch_disable_digital_output(const uint8_t pin);
   */
 void watch_set_pin_level(const uint8_t pin, const bool level);
 /// @}
+#endif