wifi: ath12k: Add missing include of export.h

wifi: ath12k: Add missing include of export.h

Commit a934a57a42f6 ("scripts/misc-check: check missing #include
<linux/export.h> when W=1") introduced a new check that is producing
the following warning:

drivers/net/wireless/ath/ath12k/core.c: warning: EXPORT_SYMBOL() is used, but #include <linux/export.h> is missing

Add the missing #include to satisfy the check.

Reviewed-by: Vasanthakumar Thiagarajan <vasanthakumar.thiagarajan@oss.qualcomm.com>
Link: https://patch.msgid.link/20250611-ath-unused-export-v1-5-c36819df7e7b@oss.qualcomm.com
Signed-off-by: Jeff Johnson <jeff.johnson@oss.qualcomm.com>