Fix build with glibc 2.36

[benjamb]

glibc 2.36 has added the pidfd_open function, if sys/pidfd.h exists then
we should include it.

Fixes the following error:

../common/common-pidfds.c: In function ‘open_pidfds’:
../common/common-pidfds.c:70:26: error: implicit declaration of function ‘pidfd_open’; did you mean ‘fdopen’? [-Werror=implicit-function-declaration]
   70 |                 int fd = pidfd_open(pid, 0);
      |                          ^~~~~~~~~~
      |                          fdopen

[benjamb]

Oops, looks like I missed #379.