dlopen.3: Change arguments to main() to "void" in EXAMPLE

Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>

man3/dlopen.3

@@ -508,7 +508,7 @@ Load the (glibc) math library, and print the cosine of 2.0:
 #include <gnu/lib-names.h>  /* Defines LIBM_SO (which will be a
                                string such as "libm.so.6") */
 int
-main(int argc, char **argv)
+main(void)
 {
     void *handle;
     double (*cosine)(double);