build: include usleep gnulib module

Without this module, attempts to sleep for 1 or more seconds
on mingw instead become a no-delay no-op.

* bootstrap.conf (gnulib_modules): Add usleep.

bootstrap.conf

@@ -58,6 +58,7 @@ sys_stat
 time_r
 timegm
 useless-if-before-free
+usleep
 vasprintf
 verify
 vc-list-files