selftests/bpf: Fix include of <sys/fcntl.h>

Update ns_current_pid_tgid.c to use '#include <fcntl.h>' and avoid compile
error against mips64el/musl libc:

  In file included from .../prog_tests/ns_current_pid_tgid.c:14:
  .../include/sys/fcntl.h:1:2: error: #warning redirecting incorrect #include <sys/fcntl.h> to <fcntl.h> [-Werror=cpp]
      1 | #warning redirecting incorrect #include <sys/fcntl.h> to <fcntl.h>
        |  ^~~~~~~
  cc1: all warnings being treated as errors

Fixes: 09c02d55 ("bpf, selftests: Fold test_current_pid_tgid_new_ns into test_progs.")
Signed-off-by: Tony Ambardar <tony.ambardar@gmail.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/8bdc869749177b575025bf69600a4ce591822609.1721713597.git.tony.ambardar@gmail.com

tools/testing/selftests/bpf/prog_tests/ns_current_pid_tgid.c

@@ -11,7 +11,7 @@
 #include <sched.h>
 #include <sys/wait.h>
 #include <sys/mount.h>
-#include <sys/fcntl.h>
+#include <fcntl.h>
 #include "network_helpers.h"
 
 #define STACK_SIZE (1024 * 1024)