Make gdbpy_parameter static

While working on the next patch in this series, I noticed that
gdbpy_parameter did not need to be exported.  This makes it "static".

2016-06-23  Tom Tromey  <tom@tromey.com>

	* python/python.c (gdbpy_parameter): Now static.
	* python/python-internal.h (gdbpy_parameter): Don't declare.

gdb/python/python.c

@@ -554,7 +554,7 @@ gdbpy_parameter_value (enum var_types type, void *var)
 /* A Python function which returns a gdb parameter's value as a Python
    value.  */
 
-PyObject *
+static PyObject *
 gdbpy_parameter (PyObject *self, PyObject *args)
 {
   struct gdb_exception except = exception_none;