backtrace.3: Small fixes to example program

Reported-by: Martin Gebert <Murphy.Gebert@gmx.de>
Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>

man3/backtrace.3

@@ -215,8 +215,8 @@ void
 myfunc3(void)
 {
     int j, nptrs;
-#define SIZE 100
-    void *buffer[100];
+    const int SIZE = 100;
+    void *buffer[SIZE];
     char **strings;
 
     nptrs = backtrace(buffer, SIZE);