app/test: fix build with musl libc

Fix following build error with musl libc:

app/test/test_eal_flags.c:152:55: error:
'O_RDONLY' undeclared (first use in this function)
      fd = openat(dirfd(hugepage_dir), dirent->d_name, O_RDONLY);
                                                       ^~~~~~~~

Fixes: 45f1b6e8680a ("app: add new tests on eal flags")
Cc: stable@dpdk.org

Signed-off-by: Natanael Copa <ncopa@alpinelinux.org>

app/test/test_eal_flags.c

@@ -17,6 +17,7 @@
 #include <sys/wait.h>
 #include <sys/file.h>
 #include <limits.h>
+#include <fcntl.h>
 
 #include <rte_per_lcore.h>
 #include <rte_debug.h>