Added missing file into test separate_subprog_assert_check

The missing file was not apparent because of the XFAIL field in
the test.opt file.

Author: tjj2017

testsuite/gnat2goto/tests/separate_subprog_assert_check/p-inc.asu

@@ -0,0 +1,7 @@
+separate (P)
+procedure Inc (N : in out Integer) is
+   Old_N : constant Integer := N;
+begin
+   N := N + 1;
+   pragma Assert (N = Old_N + 1);
+end Inc;