soc: qcom: cmd-db: Fix compilation error when CMD_DB is disabled

If CONFIG_QCOM_COMMAND_DB=n the following compilation errors will be
seen. Fix this by including the appropriate linux headers.

./include/soc/qcom/cmd-db.h: In function ‘cmd_db_read_aux_data’:
./include/soc/qcom/cmd-db.h: error: implicit declaration of function ‘ERR_PTR’;
Reviewed-by: Stephen Boyd <sboyd@kernel.org>
Signed-off-by: Sibi Sankar <sibis@codeaurora.org>
Link: https://lore.kernel.org/r/20200227125615.4727-1-sibis@codeaurora.orgSigned-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>

include/soc/qcom/cmd-db.h

@@ -4,6 +4,7 @@
 #ifndef __QCOM_COMMAND_DB_H__
 #define __QCOM_COMMAND_DB_H__
 
+#include <linux/err.h>
 
 enum cmd_db_hw_type {
 	CMD_DB_HW_INVALID = 0,