BUILD: fcgi: fix the struct name of fcgi_flt_ctx

BUILD: fcgi: fix the struct name of fcgi_flt_ctx

The struct was mistakenly spelled flt_fcgi_ctx() in fcgi_flt_stop()
when it was introduced in 2.1 with commit 78fbb9f991 ("MEDIUM:
fcgi-app: Add FCGI application and filter"), causing build issues
when trying to get the alignment of the object in pool_free() for
debugging purposes. No backport is needed as it's just used to convey
a pointer.