drivers: sensor: akm09918c: fix decoder api define

Decoder api was not defined and lead to build error
if it is used in userland.
Fix that by adding necessary DT_DRV_COMPAT #define
in akm09918c sensor decoder.

Signed-off-by: Florian Weber <[email protected]>

Author: FlorianWeber1018

drivers/sensor/asahi_kasei/akm09918c/akm09918c_decoder.c

@@ -5,6 +5,8 @@
 
 #include "akm09918c.h"
 
+#define DT_DRV_COMPAT asahi_kasei_akm09918c
+
 static int akm09918c_decoder_get_frame_count(const uint8_t *buffer,
 					     struct sensor_chan_spec chan_spec,
 					     uint16_t *frame_count)