signal: Move copy_siginfo_to_user to <linux/signal.h>

Having it in asm-generic/siginfo.h doesn't make any sense as it is in no way
architecture specific.  Move it to signal.h instead where several related
functions already reside.
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: linux-arch@vger.kernel.org
Cc: Fenghua Yu <fenghua.yu@intel.com>
Cc: Tony Luck <tony.luck@intel.com>
Cc: linux-ia64@vger.kernel.org
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: sparclinux@vger.kernel.org
Cc: "David S. Miller" <davem@davemloft.net>
Link: http://lkml.kernel.org/r/20170603190102.28866-5-hch@lst.de

include/asm-generic/siginfo.h

@@ -14,8 +14,4 @@
 #define __SI_SYS	(7 << 16)
 #define __SI_CODE(T,N)	((T) | ((N) & 0xffff))
 
-struct siginfo;
-
-extern int copy_siginfo_to_user(struct siginfo __user *to, const struct siginfo *from);
-
 #endif

include/linux/signal.h

@@ -19,6 +19,8 @@ static inline void copy_siginfo(struct siginfo *to, struct siginfo *from)
 		memcpy(to, from, __ARCH_SI_PREAMBLE_SIZE + sizeof(from->_sifields._sigchld));
 }
 
+int copy_siginfo_to_user(struct siginfo __user *to, const struct siginfo *from);
+
 /*
  * Define some primitives to manipulate sigset_t.
  */