Suppress sparse warnings for global variables initialized in headers.

Suppress sparse warnings for global variables initialized in headers.

sparse warns if a non-static variable with external linkage has an
initializer at first declaration, because it suspects that it should be
static instead.  Generally it's correct, but not in these cases, so add
a redundant declaration to suppress the warning.

The suppress warnings look like:
../ofproto/connmgr.c:40:1: warning: symbol 'VLM_connmgr' was not declared. Should it be static?
../ofproto/collectors.c:31:1: warning: symbol 'vlog_module_ptr_collectors' was not declared. Should it be static?
../ofproto/connmgr.c:43:1: warning: symbol 'counter_ofconn_stuck' was not declared. Should it be static?