Workaround string-plus-int warning

The upcoming compiler warns against adding string and int:
In file included from bionic/libc/bionic/strsignal.cpp:41:
  bionic/libc/private/bionic_sigdefs.h:58:1: error: adding 'int' to a string does not append to the string [-Werror,-Wstring-plus-int]
  __BIONIC_SIGDEF(SIGWINCH,  "Window size changed")
  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  bionic/libc/bionic/strsignal.cpp:40:83: note: expanded from macro '__BIONIC_SIGDEF'
  #define __BIONIC_SIGDEF(signal_number, unused) [ signal_number ] = #signal_number + 3,
                                                                     ~~~~~~~~~~~~~~~^~~

Use array indexing index to avoid this warning.

Test: m checkbuild
Change-Id: Ib5e20edbf5bac76352df0484dd233d0621beb4e9