add deprecated capabilities functions

these were left in glibc for binary compatibility after the public
part of the interface was removed, and libcap kept using them (with
its own copy of the header files) rather than just making the syscalls
directly. might as well add them since they're so small...

src/linux/cap.c

+#include "syscall.h"
+
+int capset(void *a, void *b)
+{
+	return syscall(SYS_capset, a, b);
+}
+
+int capget(void *a, void *b)
+{
+	return syscall(SYS_capget, a, b);
+}