testsuite: Define _POSIX_SOURCE for tests [PR113278]

As the tests assume that fileno() is visible (only part of POSIX),
define the guard to ensure that it's visible.  Currently, glibc appears
to always have this defined in C++, newlib does not.

Without this patch, fails like this can be seen:

Testing analyzer/fileno-1.c,  -std=c++98
.../fileno-1.c: In function 'int test_pass_through(FILE*)':
.../fileno-1.c:5:10: error: 'fileno' was not declared in this scope
FAIL: c-c++-common/analyzer/fileno-1.c  -std=c++98 (test for excess errors)

Patch has been verified on Linux.

gcc/testsuite/ChangeLog:
	PR testsuite/113278
	* c-c++-common/analyzer/fileno-1.c: Define _POSIX_SOURCE.
	* c-c++-common/analyzer/flex-with-call-summaries.c: Same.
	* c-c++-common/analyzer/flex-without-call-summaries.c: Same.

Signed-off-by: Torbjörn SVENSSON <torbjorn.svensson@foss.st.com>