src/tools.c: fix compilation on Windows

On Windows nanosleep() is not available and compiling fails with the
following error:

  CMakeFiles/am7xxx.dir/objects.a(tools.c.obj):tools.c:(.text+0x4a): undefined reference to `nanosleep'
  collect2: error: ld returned 1 exit status
  make[2]: *** [src/CMakeFiles/am7xxx.dir/build.make:153: lib/libam7xxx.dll] Error 1
  make[1]: *** [CMakeFiles/Makefile2:189: src/CMakeFiles/am7xxx.dir/all] Error 2
  make: *** [Makefile:130: all] Error 2

Using the native Windows function Sleep() fixes the error.