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: 575a0ae9 ("selftests: add tests for pidfd_send_signal()")
Signed-off-by: Christian Brauner <christian@brauner.io>