libc: Change the return type of strerror from "const char *" to "char *"

to follow up the spec here:
https://pubs.opengroup.org/onlinepubs/9699919799/functions/strerror.html

Signed-off-by: Xiang Xiao <xiaoxiang@xiaomi.com>

libs/libc/string/lib_strerror.c

@@ -38,8 +38,8 @@
 
 struct errno_strmap_s
 {
-  uint8_t     errnum;
-  const char *str;
+  uint8_t   errnum;
+  FAR char *str;
 };
 
 /****************************************************************************
@@ -341,7 +341,7 @@ static const struct errno_strmap_s g_errnomap[] =
  * Name: strerror
  ****************************************************************************/
 
-FAR const char *strerror(int errnum)
+FAR char *strerror(int errnum)
 {
 #ifdef CONFIG_LIBC_STRERROR
   int ndxlow = 0;