util: don't require libcap when building libsystemd-shared

src/shared/util.c includes <sys/capability.h> but doesn't use anything
defined there.  Since <sys/capability.h> is part of libcap, not libc,
don't require it.

Allows systemd-without-udevd to require fewer external libraries.

src/shared/util.c

@@ -50,7 +50,6 @@
 #include <linux/kd.h>
 #include <dlfcn.h>
 #include <sys/wait.h>
-#include <sys/capability.h>
 #include <sys/time.h>
 #include <glob.h>
 #include <grp.h>