perf tests bp_account: Make global variable static

To fix the build with newer gccs, that without this patch exit with:

LD /tmp/build/perf/tests/perf-in.o
ld: /tmp/build/perf/tests/bp_account.o:/git/perf/tools/perf/tests/bp_account.c:22: multiple definition of `the_var'; /tmp/build/perf/tests/bp_signal.o:/git/perf/tools/perf/tests/bp_signal.c:38: first defined here
make[4]: *** [/git/perf/tools/build/Makefile.build:145: /tmp/build/perf/tests/perf-in.o] Error 1

First noticed in fedora:rawhide/32 with:

[perfbuilder@a5ff49d6e6e4 ~]$ gcc --version
gcc (GCC) 10.0.1 20200216 (Red Hat 10.0.1-0.8)

Reported-by: Jiri Olsa <[e-mail]>
Cc: Adrian Hunter <[e-mail]>
Cc: Namhyung Kim <[e-mail]>
Signed-off-by: Arnaldo Carvalho de Melo <[e-mail]>

tools/perf/tests/bp_account.c

···
 #include "../perf-sys.h"
 #include "cloexec.h"
 
-volatile long the_var;
+static volatile long the_var;
 
 static noinline int test_function(void)
 {