testing/mm/cachetest/cachetest_main.c: fix compilation error

fix compilation error:

cachetest_main.c: In function ‘cachetest_main’:
cachetest_main.c:240:7: error: implicit declaration of function ‘up_flush_dcache’ [-Wimplicit-function-declaration]
  240 |       up_flush_dcache((uintptr_t)info.waddr, (uintptr_t)info.waddr +

Signed-off-by: raiden00pl <raiden00@railab.me>

Author: raiden00pl

testing/mm/cachetest/cachetest_main.c

@@ -25,6 +25,8 @@
  ****************************************************************************/
 
 #include <nuttx/config.h>
+#include <nuttx/cache.h>
+
 #include <stdio.h>
 #include <syslog.h>