tests: fix pidfd-test compilation

Define __NR_pidfd_send_signal if it isn't to prevent a potential
compilation error.

To make pidfd-test compile on all arches, irrespective of whether
or not syscall numbers are assigned, define the syscall number to -1.
If it isn't defined this will cause the kernel to return -ENOSYS.

Fixes: 575a0ae974 ("selftests: add tests for pidfd_send_signal()")
Signed-off-by: Christian Brauner <christian@brauner.io>

tools/testing/selftests/pidfd/pidfd_test.c

@@ -16,6 +16,10 @@
 
 #include "../kselftest.h"
 
+#ifndef __NR_pidfd_send_signal
+#define __NR_pidfd_send_signal -1
+#endif
+
 static inline int sys_pidfd_send_signal(int pidfd, int sig, siginfo_t *info,
 					unsigned int flags)
 {