[process] Mark process descriptor as static in PERMANENT_PROCESS

There is no need for the process descriptor to be a global variable.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/include/ipxe/process.h

  *
  */
 #define PERMANENT_PROCESS( name, step )					      \
-struct process_descriptor name ## _desc = PROC_DESC_PURE ( step );	      \
+static struct process_descriptor name ## _desc = PROC_DESC_PURE ( step );     \
 struct process name __permanent_process = {				      \
 	.list = LIST_HEAD_INIT ( name.list ),				      \
 	.desc = & name ## _desc,					      \