testsuite: refine gcc.dg/analyzer/fd-4.c test for newlib

Contrary to glibc, including stdio.h from newlib defines mode_t which
conflicts with the test's type definition.

.../gcc/testsuite/gcc.dg/analyzer/fd-4.c:19:3: error: redefinition of typedef 'mode_t' with different type
...
.../include/sys/types.h:189:25: note: previous declaration of 'mode_t' with type 'mode_t' {aka 'unsigned int'}

Defining _MODE_T_DECLARED skips the type definition.

gcc/testsuite/ChangeLog:

	* gcc.dg/analyzer/fd-4.c: Fix for newlib.