[testsuite] Make the Wnonnull independent of system headers.

[testsuite] Make the Wnonnull independent of system headers.

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