use symbol named "type" from sdl2-ffi package, not the common lisp one

otherwise sdl2-examples:basic-test is failing with: 

Undefined slot TYPE in foreign type SDL-EVENT.
   [Condition of type SIMPLE-ERROR]

Restarts:
 0: [RETRY] Retry SLIME REPL evaluation request.
 1: [*ABORT] Return to SLIME's top level.
 2: [ABORT] Abort thread (#<THREAD "repl-thread" RUNNING {1004E20063}>)

Backtrace:
  0: (CFFI::GET-SLOT-INFO SDL2-FFI:SDL-EVENT TYPE)
  1: (FOREIGN-SLOT-VALUE #.(SB-SYS:INT-SAP #X7FFFF2692AD0) SDL2-FFI:SDL-EVENT TYPE)
  2: (GET-EVENT-TYPE #.(SB-SYS:INT-SAP #X7FFFF2692AD0))
  3: (SDL2-EXAMPLES:BASIC-TEST)
  4: (SB-INT:SIMPLE-EVAL-IN-LEXENV (SDL2-EXAMPLES:BASIC-TEST) #<NULL-LEXENV>)
  5: (EVAL (SDL2-EXAMPLES:BASIC-TEST))
 --more--