'pg_addr' is only used when CONFIG_X86 is defined.  So only declare it if
CONFIG_X86 is defined.

Fixes the following W=1 kernel build warning(s):

 drivers/scsi/lpfc/lpfc_sli.c: In function ‘lpfc_wq_create’:
 drivers/scsi/lpfc/lpfc_sli.c:15813:16: warning: unused variable ‘pg_addr’ [-Wunused-variable]
 15813 | unsigned long pg_addr;
 | ^~~~~~~

Link: https://lore.kernel.org/r/20200721164148.2617584-37-lee.jones@linaro.org
Cc: James Smart <james.smart@broadcom.com>
Cc: Dick Kennedy <dick.kennedy@broadcom.com>
Cc: Sumit Semwal <sumit.semwal@linaro.org>
Cc: "Christian König" <christian.koenig@amd.com>
Cc: linux-media@vger.kernel.org
Cc: dri-devel@lists.freedesktop.org
Cc: linaro-mm-sig@lists.linaro.org
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>