netfilter: nf_conntrack_pptp: fix compilation warning with W=1 build

>> include/linux/netfilter/nf_conntrack_pptp.h:13:20: warning: 'const' type qualifier on return type has no effect [-Wignored-qualifiers]
extern const char *const pptp_msg_name(u_int16_t msg);
^~~~~~

Reported-by: kbuild test robot <lkp@intel.com>
Fixes: 4c559f15

 ("netfilter: nf_conntrack_pptp: prevent buffer overflows in debug code")
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>