sparc: include config.h to start.S

arch/sparc/cpu/leon3/start.S requires CONFIG_SYS_SPARC_NWINDOES
to be defined:

  #ifndef CONFIG_SYS_SPARC_NWINDOWS
  #error Must define number of SPARC register windows, default is 8
  #endif

But it missed to include <config.h>, which always ended up in compile error.

This commit fixes this problem.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Cc: Daniel Hellstrom <daniel@gaisler.com>