scsi: zfcp: drop unnecessary forward prototype for struct zfcp_reqlist

While struct zfcp_adapter contains a pointer to zfcp_reqlist, the pointer
field does not need to know the structure or even a prototype.

The prototype was introduced with v2.6.34 commit b6bd2fb9

 ("[SCSI]
zfcp: Move FSF request tracking code to new file").

Signed-off-by: Steffen Maier <maier@linux.ibm.com>
Reviewed-by: Benjamin Block <bblock@linux.ibm.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>