net/DCCP: fix link error with !CONFIG_SYSCTL

Quoting Ingo Molnar:

|  Subject: net/DCCP: fix link error with !CONFIG_SYSCTL
|  
|  do not define the sysctl_dccp_sync_ratelimit sysctl variable in
|  the CONFIG_SYSCTL dependent sysctl.c module - move it to input.c instead.
|  
|  this fixes the following build bug:
|  
|   net/built-in.o: In function `dccp_check_seqno':
|   input.c:(.text+0xbd859): undefined reference to `sysctl_dccp_sync_ratelimit'

[thanks for spotting, copied this to dccp@vger instead of linux-kernel]

NB: Arnaldo, in the test tree this problem is fixed and may serve as a suitable solution
for other sysctl variables also: the variables are always present, the #error wrapper has been replaced by an #ifdef, and the dccp-$(CONFIG_SYSCTL) += sysctl.o in the Makefile has been shifted to dccp-y; so that the variables are always there, independent of whether CONFIG_SYSCTL is enabled.

|  +++ linux/net/dccp/sysctl.c
|  @@ -18,9 +18,6 @@
|   #error This file should not be compiled without CONFIG_SYSCTL defined
    ^
    +------------------ this #error has been replaced by a wrapper