missing_syscall: mandate __NR_memfd_create

Follow-up for 6db5a6e799

For basic system functionality we require memfd nowadays,
so fail the build if syscall number not available.

src/basic/missing_syscall.h

@@ -80,12 +80,7 @@ static inline int missing_ioprio_set(int which, int who, int ioprio) {
 
 #if !HAVE_MEMFD_CREATE
 static inline int missing_memfd_create(const char *name, unsigned int flags) {
-#  ifdef __NR_memfd_create
         return syscall(__NR_memfd_create, name, flags);
-#  else
-        errno = ENOSYS;
-        return -1;
-#  endif
 }
 
 #  define memfd_create missing_memfd_create