gdb.base/info-os.c: Include stdlib.h

gdb.base/info-os.c: Include stdlib.h

Fixes:

 > gdb compile failed, /gdb/testsuite/gdb.base/info-os.c: In function 'main':
 > /gdb/testsuite/gdb.base/info-os.c:65:3: warning: implicit declaration of function 'atexit' [-Wimplicit-function-declaration]
 >    atexit (ipc_cleanup);
 >    ^
 > FAIL: gdb.base/info-os.exp: cannot compile test program

with recent GCCs.

gdb/testsuite/ChangeLog:
2015-02-23  Pedro Alves  <palves@redhat.com>

	* gdb.base/info-os.c: Include stdlib.h.