Add missing pxd for test.

tests/run/cpdef_enums.pxd

+cdef extern from *:
+    cpdef enum ExternPxdEnum:
+        FOUR "4"
+        EIGHT "8"
+
+    cdef enum ExternSecretPxdEnum:
+        SIXTEEN "16"
+
+cpdef enum PxdEnum:
+    RANK_0 = 11
+    RANK_1 = 37
+    RANK_2 = 389
+
+cdef enum PxdSecretEnum:
+    RANK_3 = 5077