Fix up the NumPy warnings hack by using the actual "import_array()" function...

Fix up the NumPy warnings hack by using the actual "import_array()" function declaration instead of a wrong one.

tests/run/numpy_common.pxi

 # hack to avoid C compiler warnings about unused functions in the NumPy header files
 
+from numpy cimport import_array  # , import_umath
+
 cdef extern from *:
    bint FALSE "0"
-   void import_array()
-#   void import_umath()
 
 if FALSE:
     import_array()