meson: check string.h not strings.h for explicit_bzero

I used strings.h because that's what explicit_bzero(3) says. But glibc defines
it in string.h. There are no other available implementations atm (musl does not
define it, bionic aliases it to memset in openbsd-compat.h, yikes). We also
include just string.h from the code. So just look for the function in string.h,
and if it ever appears in other places, we can change the detection and includes.

meson.build

@@ -388,7 +388,7 @@ foreach ident : [
                                  #include <keyutils.h>'''],
         ['copy_file_range',   '''#include <sys/syscall.h>
                                  #include <unistd.h>'''],
-        ['explicit_bzero' ,   '''#include <strings.h>'''],
+        ['explicit_bzero' ,   '''#include <string.h>'''],
 ]
 
         have = cc.has_function(ident[0], prefix : ident[1])