Fix SEQUENCER_ENABLE so the sequencer feature builds again (#26322)

* Make the sequencer feature build again

SEQUENCER_ENABLE only set MUSIC_ENABLE, so -DSEQUENCER_ENABLE was never
defined and neither sequencer.c nor process_sequencer.c were compiled;
process_sequencer.c also lost the includes it depends on. Wire the make
target up like the other features and add the missing includes.

* Include sequencer.h in keyboard.c for sequencer_task()

* Update builddefs/common_features.mk

Co-authored-by: Joel Challis <git@zvecr.com>

---------

Co-authored-by: Joel Challis <git@zvecr.com>

quantum/keyboard.c

@@ -150,6 +150,9 @@ along with this program.  If not, see <http://www.gnu.org/licenses/>.
 #ifdef CONNECTION_ENABLE
 #    include "connection.h"
 #endif
+#ifdef SEQUENCER_ENABLE
+#    include "sequencer.h"
+#endif
 
 static uint32_t last_input_modification_time = 0;
 uint32_t        last_input_activity_time(void) {

quantum/process_keycode/process_sequencer.c

@@ -15,6 +15,8 @@
  */
 
 #include "process_sequencer.h"
+#include "quantum_keycodes.h"
+#include "sequencer.h"
 
 bool process_sequencer(uint16_t keycode, keyrecord_t *record) {
     if (record->event.pressed) {