To avoid the result of this test depending on the implementation of
the system 'string.h', provide prototypes for the two functions used
in the test.

gcc/testsuite/ChangeLog:

2019-10-22  Iain Sandoe  <iain@sandoe.co.uk>

	* gcc.dg/Wnonnull.c: Provide prototypes for strlen and memcpy.
	Use __SIZE_TYPE__ instead of size_t.

From-SVN: r277280