perf build: Avoid defining _FORTIFY_SOURCE multiple times

One in perf's CFLAGS and the other in the distro python binding
scripts.

So if use the usual technique of first -D_FORTIFY_SOURCE then -D it.

Noticed with:

opensuse tumbleweed:

  gcc version 12.1.1 20220629 [revision 7811663964aa7e31c3939b859bbfa2e16919639f] (SUSE Linux)
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/Makefile.config

@@ -342,7 +342,7 @@ endif
 
 ifeq ($(DEBUG),0)
   ifeq ($(feature-fortify-source), 1)
-    CORE_CFLAGS += -D_FORTIFY_SOURCE=2
+    CORE_CFLAGS += -U_FORTIFY_SOURCE -D_FORTIFY_SOURCE=2
   endif
 endif