gdb: fix build failure in amd-dbgapi-target.c

Since b080fe54fb3 "gdb: add inferior-specific breakpoints", the
breakpoint class has an "inferior" member used to handle
inferior-specific breakpoints.  This creates a compilation error
in amd_dbgapi_target_breakpoint::check_status which declares a local
variable "inferior *inf".

Fix this by using "struct inferior *inf" instead.

Change-Id: Icc4dc1ba96c7d3ff9d33f9cb384ffcf64eba26fb
Approved-By: Pedro Alves <pedro@palves.net>