Blackfin: fix elf_fpregset_t definition

The elf_fpregset_t type relied on an empty struct in the asm/user.h, but
the transition to asm-generic/user.h dropped that empty struct.  Rather
than restore this useless struct, define the only user (elf_fpregset_t)
as an empty struct itself.  This fixes building when ELF dump support is
enabled.
Signed-off-by: Mike Frysinger <vapier@gentoo.org>

arch/blackfin/include/asm/elf.h

@@ -23,7 +23,7 @@ typedef unsigned long elf_greg_t;
 #define ELF_NGREG 40 /* (sizeof(struct user_regs_struct) / sizeof(elf_greg_t)) */
 typedef elf_greg_t elf_gregset_t[ELF_NGREG];
 
-typedef struct user_bfinfp_struct elf_fpregset_t;
+typedef struct { } elf_fpregset_t;
 /*
  * This is used to ensure we don't load something for the wrong architecture.
  */