Staging: hv: hv_mouse: fix camelcase use of struct synthhid_msg_hdr

s/Header/header/g for this structure when it is used in the file.

Cc: Hank Janssen <hjanssen@microsoft.com>
Cc: K. Y. Srinivasan <kys@microsoft.com>
Cc: Haiyang Zhang <haiyangz@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>