libc: Add missing header for memset

Silence compiler warning by adding the missing string.h header
to include memset prototype.
Do some code tidy-up removing commented out libc_hidden_proto macros
in the meanwhile.

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

libc/signal/sigblock.c

@@ -19,8 +19,6 @@
 #define __UCLIBC_HIDE_DEPRECATED__
 #include <errno.h>
 #include <signal.h>
-#include <string.h>
-
 
 #include "sigset-cvt-mask.h"
 

libc/signal/sigpause.c

@@ -27,7 +27,6 @@
 #include <sysdep-cancel.h>
 #endif
 
-
 #include "sigset-cvt-mask.h"
 
 /* Set the mask of blocked signals to MASK,

libc/signal/sigsetmask.c

@@ -19,8 +19,6 @@
 #define __UCLIBC_HIDE_DEPRECATED__
 #include <errno.h>
 #include <signal.h>
-#include <string.h>
-
 
 #include "sigset-cvt-mask.h"