hurd: fix RPC stubs names

[Samuel Thibault]

Mach RPCs expect C names for RPC stubs, so we should make sure they are
defined with C names.

* gdb/gnu-nat.c (ILL_RPC): Enclose RPC stub definition in extern "C" { }

diff --git a/gdb/gnu-nat.c b/gdb/gnu-nat.c
index d5e3841e68..b39d3d0a90 100644
--- a/gdb/gnu-nat.c
+++ b/gdb/gnu-nat.c
@@ -1796,11 +1796,13 @@ do_mach_notify_dead_name (mach_port_t notify, 
mach_port_t dead_port)
 
 
 #define ILL_RPC(fun, ...) \
+  extern "C" { \
   extern kern_return_t fun (__VA_ARGS__); \
   kern_return_t fun (__VA_ARGS__) \
   { \
     warning (_("illegal rpc: %s"), #fun); \
     return 0; \
+  } \
   }
 
 ILL_RPC (do_mach_notify_no_senders,