libc: Remove pointless header string.h

It is already include from the internal header sigset-cvt-mask.h

Signed-off-by: Carmelo Amoroso <carmelo.amoroso@st.com>

libc/signal/sigpause.c

@@ -23,7 +23,6 @@
 #define __FAVOR_BSD
 #include <signal.h>
 #include <stddef.h>		/* For NULL.  */
-#include <string.h>
 
 #include "sigset-cvt-mask.h"