signal: remove duplicate include in signal.h

'linux/string.h' included in 'signal.h' is duplicated.
it's also included at line 7.

Link: https://lkml.kernel.org/r/20211019024934.973008-1-ye.guojin@zte.com.cnSigned-off-by: Ye Guojin <ye.guojin@zte.com.cn>
Reported-by: Zeal Robot <zealci@zte.com.cn>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/signal.h

@@ -126,7 +126,6 @@ static inline int sigequalsets(const sigset_t *set1, const sigset_t *set2)
 #define sigmask(sig)	(1UL << ((sig) - 1))
 
 #ifndef __HAVE_ARCH_SIG_SETOPS
-#include <linux/string.h>
 
 #define _SIG_SET_BINOP(name, op)					\
 static inline void name(sigset_t *r, const sigset_t *a, const sigset_t *b) \