proc-pidns.c uses open() and O_* flags, but does not include
<fcntl.h>. This breaks the proc selftests build with errors such as:

  error: implicit declaration of function 'open'
  error: 'O_WRONLY' undeclared
  error: 'O_CREAT' undeclared
  error: 'O_RDONLY' undeclared

Include <fcntl.h> to provide the declaration and flag definitions.

Fixes: 5554d820f71c ("selftests/proc: add tests for new pidns APIs")
Tested with:
  make -C tools/testing/selftests TARGETS=proc

Signed-off-by: Amin Vakil <info@aminvakil.com>
---
 tools/testing/selftests/proc/proc-pidns.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/tools/testing/selftests/proc/proc-pidns.c b/tools/testing/selftests/proc/proc-pidns.c
index 25b9a2933c45..6f7c10fe97b3 100644
--- a/tools/testing/selftests/proc/proc-pidns.c
+++ b/tools/testing/selftests/proc/proc-pidns.c
@@ -6,6 +6,7 @@
 
 #include <assert.h>
 #include <errno.h>
+#include <fcntl.h>
 #include <sched.h>
 #include <stdbool.h>
 #include <stdlib.h>
-- 
2.54.0