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>

include/linux/netfilter/nf_conntrack_pptp.h

@@ -10,7 +10,7 @@
 #include <net/netfilter/nf_conntrack_expect.h>
 #include <uapi/linux/netfilter/nf_conntrack_tuple_common.h>
 
-extern const char *const pptp_msg_name(u_int16_t msg);
+const char *pptp_msg_name(u_int16_t msg);
 
 /* state of the control session */
 enum pptp_ctrlsess_state {

net/netfilter/nf_conntrack_pptp.c

@@ -91,7 +91,7 @@ static const char *const pptp_msg_name_array[PPTP_MSG_MAX + 1] = {
 	[PPTP_SET_LINK_INFO]		= "SET_LINK_INFO"
 };
 
-const char *const pptp_msg_name(u_int16_t msg)
+const char *pptp_msg_name(u_int16_t msg)
 {
 	if (msg > PPTP_MSG_MAX)
 		return pptp_msg_name_array[0];