testsuits/dl10 : Prototype missing

Fix a compiler warning about a missing prototype.

If you wonder why the function name is not `static`:
The code in the file only serves a test where the dynamic object loader
(aka dlopen()) should handle duplicated symbols in an archive.

testsuites/libtests/dl10/dl10-o6.c:14:5: warning: no previous prototype
for 'rtems_main_o5' [-Wmissing-prototypes]
       14 | int rtems_main_o5 (void)
          |     ^~~~~~~~~~~~~

testsuites/libtests/dl10/dl10-o6.c

@@ -7,6 +7,7 @@
  */
 
 #include "dl-load.h"
+#include "dl-o5.h"
 
 #include <inttypes.h>
 #include <rtems/test-info.h>