Remove libc/time.pxd dependency upon sys/types.h (on suggestion of Larsmans).

Also, remove clock_t from posix.types making users find it in libc.time.  It's
really a standard C type and not used anywhere in system interfaces anyway.

Cython/Includes/libc/time.pxd

 # http://en.wikipedia.org/wiki/C_date_and_time_functions
 
-from posix.types cimport clock_t, time_t
 from libc.stddef cimport wchar_t
 
 cdef extern from "time.h" nogil:
+    ctypedef long clock_t
+    ctypedef long time_t
+
     enum: CLOCKS_PER_SEC
     clock_t clock()             # CPU time
     time_t  time(time_t *)      # wall clock time since Unix epoch

Cython/Includes/posix/types.pxd

 cdef extern from "sys/types.h":
     ctypedef long blkcnt_t
     ctypedef long blksize_t
-    ctypedef long clock_t
     ctypedef long clockid_t
     ctypedef long dev_t
     ctypedef long gid_t