linux-user: Make target_strerror() return 'const char *'

Make target_strerror() return 'const char *' rather than just 'char *';
this will allow us to return constant strings from it for some special
cases.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Riku Voipio <riku.voipio@linaro.org>
Reviewed-by: Laurent Vivier <laurent@vivier.eu>

linux-user/syscall.c

@@ -630,7 +630,7 @@ static inline int is_error(abi_long ret)
     return (abi_ulong)ret >= (abi_ulong)(-4096);
 }
 
-char *target_strerror(int err)
+const char *target_strerror(int err)
 {
     if ((err >= ERRNO_TABLE_SIZE) || (err < 0)) {
         return NULL;