perf tools: Do not use _FORTIFY_SOURCE when DEBUG=1 is specified

perf tools: Do not use _FORTIFY_SOURCE when DEBUG=1 is specified

As:

make DEBUG=1 -C tools/perf

disables optimizations and _FORTIFY_SOURCE in recent distros requires
optimizations to be enabled, seen on a Fedora 17 system:

[acme@Fedora17 linux]$ make DEBUG=1 O=/home/acme/git/build/perf/ -C
tools/perf install
In file included from /usr/include/sys/types.h:26:0,
                 from /usr/include/libelf.h:53,
                 from /usr/include/gelf.h:53,
                 from /usr/include/elfutils/libdw.h:53,
                 from <stdin>:2:
/usr/include/features.h:314:4: error: #warning _FORTIFY_SOURCE requires
compiling with optimization (-O) [-Werror=cpp

Cc: David Ahern <dsahern@gmail.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Namhyung Kim <namhyung@gmail.com>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
Link: http://lkml.kernel.org/n/tip-4ccyiebqju4uatm31ky7725b@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>